Electrophoretic electronic displays with low-index films

ABSTRACT

The invention features a reflective display device and a method of making a reflective display device that has reduced light loss and/or pixel cross talk due to internal reflection. The device includes a window layer, a plurality of reflective particles, a material portion disposed between the window layer and the plurality of reflective particles, and a refractive layer disposed between the window and the material portion. The plurality of reflective particles scatters light received from the ambient environment. The window layer has an index of refraction that is greater than an index of refraction of the ambient environment. The refractive layer has an index of refraction that is less than the index of refraction of the window layer and less than an index of refraction of the material portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to and benefit of U.S.provisional patent application serial No. 60/341,183 filed Dec. 13,2001, the entire disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

[0002] The present invention relates to displays, and, in particular, toblack and white and color electrophoretic displays.

BACKGROUND OF THE INVENTION

[0003] Electrophoretic displays typically use reflective and absorbingpigments to produce optical performance that dramatically differs fromtraditional electronic displays. The white pigments, for example, usedin the electrophoretic displays typically reflect light by a multiplescattering mechanism. The pigments thus are isotropic diffusers thatcreate a Lambertian distribution of light reflected from a pigmentsurface. Electrophoretic displays thus typically have a light outputintensity that approximates a Lambertian distribution. The output ofreflective liquid crystal displays, in contrast, has an intensity thatvaries substantially with viewing direction.

[0004] Certain optical inefficiencies exist, however, in the typicalelectrophoretic display. In a typical electrophoretic display, a sizablefraction of light scatters from the reflective pigment at a steep anglerelative to the normal to the scattering surface. A large portion ofthis scattered light then experiences internal reflection at theinterface between the ambient air and the front surface of the display.After internal reflection, the light has only a small chance of being“recycled” (i.e., reflected in a manner so as to be capable of beingseen by an observer of the display), through re-scattering by a nearbypixel.

[0005] Internal reflection inefficiency can cause the brightness of theelectrophoretic display to be reduced by up to 50% or more. Furthermore,re-scattering from neighboring pixels can cause undesirable opticalcross-talk.

SUMMARY OF THE INVENTION

[0006] The present invention provides displays with reduced opticalinefficiency. The invention features displays that include a refractivelayer of low refractive index, a reflective portion, and an intermediatematerial portion having a higher refractive index than the refractivelayer. A display medium can include the reflective portion, for example,a reflective particle, and can include an encapsulating structure,preferably thin, that in part resides between the reflective portion ofthe display medium and the low-index refraction layer. The reflectiveparticle may be an electrophoretic particle. The encapsulating structurecan be, for example, a capsule membrane or a binder.

[0007] When the encapsulating structure is thin, its interface with thelow-index refraction layer is close to the reflective portion of thedisplay medium. As a result, when light from a spot on the reflectiveportion of the display medium, for example, part of a pixel, reaches theinterface between the encapsulating structure and the low-indexrefraction layer, the portion of the light that gets internallyreflected is more likely to bounce back onto a second spot proximate tothe first spot. In an embodiment, the two spots are of the same pixel.As a result, more light gets “recycled” and eventually reaches anobserver of the display. Moreover, optical cross-talk is reduced becausethe internal reflection is more localized.

[0008] The invention can be applied to both monochrome and colorelectrophoretic displays. Moreover, the invention can be applied toother display materials or display designs that involve a Lambertian ornear-Lambertian optical response.

[0009] Features of the invention can provide Lambertian displays havingan increase in brightness of as much as 50% or more. The invention worksfor color filter-based displays as well as non-color displays.

[0010] In one embodiment, the low-index film is disposed betweenencapsulated reflecting particles and a front window of the display. Thelow-index film, however, can be disposed in a variety of otherlocations.

[0011] Accordingly, in a first aspect, the invention features a display.The display includes a window layer, a plurality of reflectiveparticles, a material portion between the particles and window, and arefractive layer between the material portion and window. The windowlayer has an index of refraction that is greater than an index ofrefraction of the ambient environment, such as air. The plurality ofreflective particles scatters light received from the ambientenvironment. The refractive layer is disposed between the display mediumlayer and the window layer. The refractive layer has an index ofrefraction that is less than the index of refraction of the window layerand less than an index of refraction of the material portion.

[0012] The reflective particles belong to a display medium layer. Thereflective particles can be a component of an unencapsulated displaymedium layer, and the material portion can assist containment of theparticles. Such a material portion can be one or more layers, forexample, a barrier layer, which can be flexible or rigid.

[0013] Alternatively, the reflective particles and material portion canbe components of an encapsulated display medium layer. The materialportion can then be a portion of the encapsulated display medium layerthat resides between the reflective particles and the refractive layer.The material portion can include, but is not limited to, for example,capsule membrane, binder, polymer film, and/or fluid.

[0014] The display can include one or more film layers between and incontact with the display medium layer and the refractive layer. Theselayers can include, for example, a capping layer and/or an electricallyconductive layer. The portion of the encapsulating structure and thefilm layers can have a combined thickness that causes most internallyreflected scattered light to return to the same pixel from which thelight is scattered. The combined thickness can be, for example, lessthan 10 micrometers, or less than 3 micrometers. The film layers canhave thicknesses in a range of, for example, 0.05 to 0.30 micrometer.

[0015] The refractive layer can include a vacuum or a gas-filled gap.Alternatively, the refractive layer can include a porous material and/ora composite material. The refractive layer can have a thickness greaterthan the longest wavelength of visible light incident upon the display.For example, the thickness can be greater than 1 micrometer.

[0016] In a second aspect, the invention features a method of making adisplay device. The method includes providing a window layer, aplurality of reflective particles, a material portion between theparticles and window, and a refractive layer between the materialportion and window. The window layer has an index of refraction that isgreater than an index of refraction of the ambient environment, such asair. The plurality of reflective particles scatters light received fromthe ambient environment. The refractive layer is disposed between thedisplay medium layer and the window layer. The refractive layer has anindex of refraction that is less than the index of refraction of thewindow layer and less than an index of refraction of the materialportion.

[0017] The method can include selecting a thickness of the materialportion to cause most internally reflected scattered light to return toa same pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention is pointed out with particularity in the appendedclaims. The advantages of the invention described above, together withfurther advantages, may be better understood by referring to thefollowing description taken in conjunction with the accompanyingdrawings. In the drawings, like reference characters generally refer tothe same parts throughout the different views. Also, the drawings arenot necessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention.

[0019]FIG. 1A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich the smaller electrode has been placed at a voltage relative to thelarge electrode causing the particles to migrate to the smallerelectrode.

[0020]FIG. 1B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich the larger electrode has been placed at a voltage relative to thesmaller electrode causing the particles to migrate to the largerelectrode.

[0021]FIG. 1C is a diagrammatic top-down view of one embodiment of arear-addressing electrode structure.

[0022]FIG. 2A is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerassociated with the larger electrode in which the smaller electrode hasbeen placed at a voltage relative to the large electrode causing theparticles to migrate to the smaller electrode.

[0023]FIG. 2B is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerassociated with the larger electrode in which the larger electrode hasbeen placed at a voltage relative to the smaller electrode causing theparticles to migrate to the larger electrode.

[0024]FIG. 2C is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerdisposed below the larger electrode in which the smaller electrode hasbeen placed at a voltage relative to the large electrode causing theparticles to migrate to the smaller electrode.

[0025]FIG. 2D is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerdisposed below the larger electrode in which the larger electrode hasbeen placed at a voltage relative to the smaller electrode causing theparticles to migrate to the larger electrode.

[0026]FIG. 3A is a diagrammatic side view of an embodiment of anaddressing structure in which a direct-current electric field has beenapplied to the capsule causing the particles to migrate to the smallerelectrode.

[0027]FIG. 3B is a diagrammatic side view of an embodiment of anaddressing structure in which an alternating-current electric field hasbeen applied to the capsule causing the particles to disperse into thecapsule.

[0028]FIG. 3C is a diagrammatic side view of an embodiment of anaddressing structure having transparent electrodes, in which adirect-current electric field has been applied to the capsule causingthe particles to migrate to the smaller electrode.

[0029]FIG. 3D is a diagrammatic side view of an embodiment of anaddressing structure having transparent electrodes, in which analternating-current electric field has been applied to the capsulecausing the particles to disperse into the capsule.

[0030]FIG. 4A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich multiple smaller electrodes have been placed at a voltage relativeto multiple larger electrodes, causing the particles to migrate to thesmaller electrodes.

[0031]FIG. 4B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich multiple larger electrodes have been placed at a voltage relativeto multiple smaller electrodes, causing the particles to migrate to thelarger electrodes.

[0032]FIG. 5A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display havingcolored electrodes and a white electrode, in which the coloredelectrodes have been placed at a voltage relative to the white electrodecausing the particles to migrate to the colored electrodes.

[0033]FIG. 5B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display havingcolored electrodes and a white electrode, in which the white electrodehas been placed at a voltage relative to the colored electrodes causingthe particles to migrate to the white electrode.

[0034]FIG. 6 is a diagrammatic side view of an embodiment of a colordisplay element having red, green, and blue particles of differentelectrophoretic mobilities.

[0035] FIGS. 7A-7B depict the steps taken to address the display of FIG.6 to display red.

[0036] FIGS. 8A-8D depict the steps taken to address the display of FIG.6 to display blue.

[0037] FIGS. 9A-9C depict the steps taken to address the display of FIG.6 to display green.

[0038]FIG. 10 is a perspective embodiment of a rear electrode structurefor addressing a seven segment display.

[0039]FIG. 11 is a perspective embodiment of a rear electrode structurefor addressing a three by three matrix display element.

[0040]FIG. 12 is a cross-sectional view of a printed circuit board usedas a rear electrode addressing structure.

[0041]FIG. 13 is a cross-sectional view of a dielectric sheet used as arear electrode addressing structure.

[0042]FIG. 14 is a cross-sectional view of a rear electrode addressingstructure that is formed by printing.

[0043]FIG. 15 is a perspective view of an embodiment of a control gridaddressing structure.

[0044]FIG. 16 is an embodiment of an electrophoretic display that can beaddressed using a stylus.

[0045]FIG. 17A is a cross-sectional view of an embodiment of anelectrophoretic display medium.

[0046]FIG. 17B is a planar-sectional view of an electrophoretic displaymedium corresponding to the embodiment shown in FIG. 17A.

[0047] FIGS. 18A-18C are cross-sectional views of embodiments of anelectrophoretic display element having an optical biasing element atvarious locations.

[0048]FIGS. 19A and 19B are cross-sectional views of embodiments of anelectrophoretic display element having an optical biasing elementembedded in a binder.

[0049]FIGS. 20A and 20B are cross-sectional views of embodiments of anelectrophoretic display element having an optical biasing elementembedded in an electrode.

[0050]FIG. 21 is a cross-sectional view of an embodiment of anelectrophoretic display.

[0051] FIGS. 22A-C are cross-sectional views of embodiments of upperlayers of electrophoretic displays including a low-index layer.

[0052]FIG. 23 is a cross-sectional view illustrating a path of incidentlight into an embodiment of an electrophoretic display including alow-index layer.

[0053]FIG. 24 is a cross-sectional view of an embodiment of anelectrophoretic display without a low-index layer, which illustratespaths of light out of the display.

[0054]FIG. 25 is a cross-sectional view of an embodiment of anelectrophoretic display with a low-index layer, which illustrates pathsof light out of the display.

DETAILED DESCRIPTION

[0055] In the following embodiments of the invention, colorelectrophoretic displays are first described, with reference to FIGS.1-16. Electrophoretic displays that include an optical biasing elementare described, with reference to FIGS. 17-20. Illustrative embodimentsof reflective displays that include a low refractive index (“low-index”)film to enhance the optical characteristics of the displays aredescribed with reference to FIGS. 21-25. It is to be recognized that allof the electrophoretic displays described herein and illustrated in thefigures may include a low-index film according to the present invention.Lastly, various embodiments of materials that may be included in anelectrophoretic display medium of the present invention are furtherdescribed.

[0056] An electronic ink is an optoelectronically active material whichcomprises at least two phases: an electrophoretic contrast medium phaseand a coating/binding phase. The electrophoretic phase comprises, insome embodiments, a single species of electrophoretic particlesdispersed in a clear or dyed medium, or more than one species ofelectrophoretic particles having distinct physical and electricalcharacteristics dispersed in a clear or dyed medium. In some embodimentsthe electrophoretic phase is encapsulated, that is, there is a capsulewall phase, i.e., a membrane, between the two phases.

[0057] The coating/binding phase includes, in one embodiment, a polymermatrix that surrounds the electrophoretic phase. In this embodiment, thepolymer in the polymeric binder is capable of being dried, crosslinked,or otherwise cured as in traditional inks, and therefore a printingprocess can be used to deposit the electronic ink onto a substrate. Anelectronic ink is capable of being printed by several differentprocesses, depending on the mechanical properties of the specific inkemployed. For example, the fragility or viscosity of a particular inkmay result in a different process selection. A very viscous ink wouldnot be well-suited to deposition by an inkjet printing process, while afragile ink might not be used in a knife over roll coating process.

[0058] The optical quality of an electronic ink is quite distinct fromother electronic display materials. The most notable difference is thatthe electronic ink provides a high degree of both reflectance andcontrast because it is pigment based (as are ordinary printing inks).The light scattered from the electronic ink comes from a very thin layerof pigment close to the top of the viewing surface. In this respect itresembles an ordinary, printed image. Also, electronic ink is easilyviewed from a wide range of viewing angles in the same manner as aprinted page, and such ink approximates a Lambertian contrast curve moreclosely than any other electronic display material.

[0059] Since electronic ink can be printed, it can be included on thesame surface with any other printed material, including traditionalinks. Electronic ink can be made optically stable in all displayconfigurations, that is, the ink can be set to a persistent opticalstate. Fabrication of a display by printing an electronic ink isparticularly useful in low power applications because of this stability.

[0060] Electronic ink displays are novel in that they can be addressedby DC voltages and draw very little current. As such, the conductiveleads and electrodes used to deliver the voltage to electronic inkdisplays can be of relatively high resistivity. The ability to useresistive conductors substantially widens the number and type ofmaterials that can be used as conductors in electronic ink displays. Inparticular, the use of costly vacuum-sputtered indium tin oxide (ITO)conductors, a standard material in liquid crystal devices, is notrequired.

[0061] Aside from cost savings, the replacement of ITO with othermaterials can provide benefits in appearance, processing capabilities(printed conductors), flexibility, and durability. Additionally, theprinted electrodes are in contact only with a solid binder, not with afluid layer (like liquid crystals). This means that some conductivematerials, which would otherwise dissolve or be degraded by contact withliquid crystals, can be used in an electronic ink application. Theseinclude opaque metallic inks for the rear electrode (e.g., silver andgraphite inks), as well as conductive transparent inks for eithersubstrate. These conductive coatings include semiconducting colloids,examples of which are indium tin oxide and antimony-doped tin oxide.

[0062] Organic conductors (polymeric conductors and molecular organicconductors) also may be used. Polymers include, but are not limited to,polyaniline and derivatives, polythiophene and derivatives,poly-3,4-ethylenedioxythiophene (PEDOT) and derivatives, polypyrrole andderivatives, and polyphenylenevinylene (PPV) and derivatives. Organicmolecular conductors include, but are not limited to, derivatives ofnaphthalene, phthalocyanine, and pentacene. Polymer layers can be madethinner and more transparent than with traditional displays becauseconductivity requirements are not as stringent.

[0063] As an example, there is a class of materials calledelectroconductive powders, which are also useful as coatable transparentconductors in electronic ink displays. One example is Zelec ECPelectroconductive powders from DuPont Chemical Co. of Wilmington, Del.

[0064] Referring now to FIGS. 1A and 1B, an addressing scheme forcontrolling particle-based displays is shown in which electrodes aredisposed on only one side of a display, allowing the display to berear-addressed. Utilizing only one side of the display for electrodessimplifies fabrication of displays. For example, if the electrodes aredisposed on only the rear side of a display, both of the electrodes canbe fabricated using opaque materials, because the electrodes do not needto be transparent.

[0065]FIG. 1A depicts a single capsule 20 of an encapsulated displaymedia. In brief overview, the embodiment depicted in FIG. 1A includes acapsule 20 containing at least one particle 50 dispersed in a suspendingfluid 25. The capsule 20 is addressed by a first electrode 30 and asecond electrode 40. The two electrodes 30 and 40 may differ in “size.”For example, the first electrode 30 may be smaller than the secondelectrode 40. The first electrode 30 and the second electrode 40 may beset to voltage potentials which affect the position of the particles 50in the capsule 20.

[0066] The particles 50 may represent 0.1% to 20% of the volume enclosedby the capsule 20. In some embodiments the particles 50 represent 2.5%to 17.5% of the volume enclosed by capsule 20. In preferred embodiments,the particles 50 represent 5% to 15% of the volume enclosed by thecapsule 20. In more preferred embodiments the particles 50 represent 9%to 11% of the volume defined by the capsule 20. In general, the volumepercentage of the capsule 20 that the particles 50 represent should beselected so that the particles 50 expose most of the second, largerelectrode 40 when positioned over the first, smaller electrode 30. Asdescribed in detail below, the particles 50 may be colored any one of anumber of colors. The particles 50 may be either positively charged ornegatively charged, or neutral in charge.

[0067] Still referring to FIGS. 1A and 1B, the particles 50 aredispersed in a dispersing fluid 25. The dispersing fluid 25 may have alow dielectric constant. The fluid 25 may be clear, or substantiallyclear, so that the fluid 25 does not inhibit viewing the particles 50and the electrodes 30, 40 from position 10. In other embodiments, thefluid 25 is dyed. In some embodiments the dispersing fluid 25 has aspecific densitymatched to the density of the particles 50. Theseembodiments can provide a bistable display media, because the particles50 do not tend to move in certain compositions absent an electric fieldapplied via the electrodes 30, 40.

[0068] The electrodes 30 and 40 should be sized and positionedappropriately so that together they address the entire capsule 20. Theremay be exactly one pair of electrodes (30, 40) addressing each capsule20, multiple pairs of electrodes (30, 40) addressing each capsule 20, ora single pair of electrodes (30, 40) may address multiple capsules 20.In the embodiment shown in FIGS. 1A and 1B, the capsule 20 has aflattened, rectangular shape. In these embodiments, the electrodes 30,40 should address most, or all, of the flattened surface area adjacentthe electrodes 30, 40.

[0069] In one embodiment, the “smaller” electrode 30 may be less thanone-half the “size” of the larger electrode 40. In preferred embodimentsthe smaller electrode is one-quarter the “size” of the larger electrode40; in more preferred embodiments the smaller electrode 30 is one-eighththe “size” of the larger electrode 40. In even more preferredembodiments, the smaller electrode 30 is one-sixteenth the “size” of thelarger electrode 40. It should be noted that reference to “smaller” inconnection with the electrode 30 means that the electrode 30 addresses asmaller amount of the surface area of the capsule 20, not necessarilythat the electrode 30 is physically smaller than the larger electrode40. For example, multiple capsules 20 may be positioned such that lessof each capsule 20 is addressed by the “smaller” electrode 30, eventhough both electrodes 30, 40 may be equal in their physical size. Itshould also be noted that, as shown in FIG. 1C, electrode 30 may addressonly a small corner of a rectangular capsule 20 (shown in phantom viewin FIG. 1C), requiring the larger electrode 40 to surround the smallerelectrode 30 on two sides in order to properly address the capsule 20.Selection of the percentage volume of the particles 50 and theelectrodes 30, 40 in this manner allow the encapsulated display media tobe addressed as described below.

[0070] Electrodes may be fabricated from any material capable ofconducting electricity so that electrode 30, 40 may apply an electricfield to the capsule 20. As noted above, the rear-addressed embodimentsdepicted in FIGS. 1A and 1B allow the electrodes 30, 40 to be fabricatedfrom opaque materials such as solder paste, copper, copper-cladpolyimide, graphite inks, silver inks and other metal-containingconductive inks. Alternatively, electrodes may be fabricated usingtransparent materials such as indium tin oxide and conductive polymerssuch as polyaniline or polythiopenes. Electrodes 30, 40 may be providedwith contrasting optical properties. In some embodiments, one of theelectrodes has an optical property complementary to optical propertiesof the particles 50.

[0071] In one embodiment, the capsule 20 contains positively chargedblack particles 50, and a substantially clear suspending fluid 25. Thefirst, smaller electrode 30 is colored black, and is smaller than thesecond electrode 40, which is colored white or is highly reflective.When the smaller, black electrode 30 is placed at a negative voltagepotential relative to larger, white electrode 40, the positively-chargedparticles 50 migrate to the smaller, black electrode 30. The effect to aviewer of the capsule 20 located at position 10 is a mixture of thelarger, white electrode 40 and the smaller, black electrode 30, creatingan effect which is largely white.

[0072] Referring to FIG. 1B, when the smaller, black electrode 30 isplaced at a positive voltage potential relative to the larger, whiteelectrode 40, particles 50 migrate to the larger, white electrode 40 andthe viewer is presented a mixture of the black particles 50 covering thelarger, white electrode 40 and the smaller, black electrode 30, creatingan effect which is largely black. In this manner the capsule 20 may beaddressed to display either a white visual state or a black visualstate.

[0073] Other two-color schemes are provided by varying the color of thesmaller electrode 30 and the particles 50 or by varying the color of thelarger electrode 40. For example, varying the color of the largerelectrode 40 allows fabrication of a rear-addressed, two-color displayhaving black as one of the colors. Alternatively, varying the color ofthe smaller electrode 30 and the particles 50 allow a rear-addressedtwo-color system to be fabricated having white as one of the colors.

[0074] Further, it is contemplated that the particles 50 and the smallerelectrode 30 can be of different colors. In these embodiments, atwo-color display may be fabricated having a second color that isdifferent from the color of the smaller electrode 30 and the particles50. For example, a rear-addressed, orange-white display may befabricated by providing blue particles 50, a red, smaller electrode 30,and a white (or highly reflective) larger electrode 40. In general, theoptical properties of the electrodes 30, 40 and the particles 50 can beindependently selected to provide desired display characteristics. Insome embodiments the optical properties of the dispersing fluid 25 mayalso be varied, e.g. the fluid 25 may be dyed.

[0075] In other embodiments, the larger electrode 40 may be reflectiveinstead of white. In these embodiments, when the particles 50 are movedto the smaller electrode 30, light reflects off the reflective surface60 associated with the larger electrode 40 and the capsule 20 appearslight in color, e.g. white (see FIG. 2A). When the particles 50 aremoved to the larger electrode 40, the reflecting surface 60 is obscuredand the capsule 20 appears dark (see FIG. 2B) because light is absorbedby the particles 50 before reaching the reflecting surface 60.

[0076] The reflecting surface 60 for the larger electrode 40 may possessretroflective properties, specular reflection properties, diffusereflective properties or gain reflection properties. In certainembodiments, the reflective surface 60 reflects light with a Lambertiandistribution. The surface 60 may be provided as a plurality of glassspheres disposed on the electrode 40, a diffractive reflecting layersuch as a holographically formed reflector, a surface patterned tototally internally reflect incident light, a brightness-enhancing film,a diffuse reflecting layer, an embossed plastic or metal film, or anyother known reflecting surface. The reflecting surface 60 may beprovided as a separate layer laminated onto the larger electrode 40 orthe reflecting surface 60 may be provided as a unitary part of thelarger electrode 40.

[0077] In the embodiments depicted by FIGS. 2C and 2D, the reflectingsurface 60 may be disposed below the electrodes 30, 40 vis-à-vis theviewpoint 10. In these embodiments, electrode 30 may be transparent sothat light may be reflected by surface 60. In other embodiments, properswitching of the particles may be accomplished with a combination ofalternating-current (AC) and direct-current (DC) electric fields anddescribed below in connection with FIGS. 3A-3D.

[0078] In still other embodiments, the rear-addressed display previouslydiscussed can be configured to transition between largely transmissiveand largely opaque modes of operation (referred to hereafter as “shuttermode”). Referring back to FIGS. 1A and 1B, in these embodiments thecapsule 20 contains at least one positively-charged particle 50dispersed in a substantially clear dispersing fluid 25. The largerelectrode 40 is transparent and the smaller electrode 30 is opaque. Whenthe smaller, opaque electrode 30 is placed at a negative voltagepotential relative to the larger, transmissive electrode 40, theparticles 50 migrate to the smaller, opaque electrode 30. The effect toa viewer of the capsule 20 located at position 10 is a mixture of thelarger, transparent electrode 40 and the smaller, opaque electrode 30,creating an effect which is largely transparent.

[0079] Referring to FIG. 1B, when the smaller, opaque electrode 30 isplaced at a positive voltage potential relative to the larger,transparent electrode 40, particles 50 migrate to the second electrode40 and the viewer is presented a mixture of the opaque particles 50covering the larger, transparent electrode 40 and the smaller, opaqueelectrode 30, creating an effect which is largely opaque. In thismanner, a display formed using the capsules depicted in FIGS. 1A and 1Bmay be switched between transmissive and opaque modes. Such a displaycan be used to construct a window that can be rendered opaque. AlthoughFIGS. 1A-2D depict a pair of electrodes associated with each capsule 20,it should be understood that each pair of electrodes may be associatedwith more than one capsule 20.

[0080] A similar technique may be used in connection with the embodimentof FIGS. 3A, 3B, 3C, and 3D. Referring to FIG. 3A, a capsule 20 containsat least one dark or black particle 50 dispersed in a substantiallyclear dispersing fluid 25. A smaller, opaque electrode 30 and a larger,transparent electrode 40 apply both direct-current (DC) electric fieldsand alternating-current (AC) fields to the capsule 20. A DC field can beapplied to the capsule 20 to cause the particles 50 to migrate towardsthe smaller electrode 30. For example, if the particles 50 arepositively charged, the smaller electrode is placed a voltage that ismore negative than the larger electrode 40. Although FIGS. 3A-3D depictonly one capsule per electrode pair, multiple capsules may be addressedusing the same electrode pair.

[0081] The smaller electrode 30 is at most one-half the “size” of thelarger electrode 40. In preferred embodiments the smaller electrode isone-quarter the “size” of the larger electrode 40; in more preferredembodiments the smaller electrode 30 is one-eighth the “size” of thelarger electrode 40. In even more preferred embodiments, the smallerelectrode 30 is one-sixteenth the “size” of the larger electrode 40.

[0082] Causing the particles 50 to migrate to the smaller electrode 30,as depicted in FIG. 3A, allows incident light to pass through thelarger, transparent electrode 40 and be reflected by a reflectingsurface 60. In shutter mode, the reflecting surface 60 is replaced by atranslucent layer, a transparent layer, or a layer is not provided atall, and incident light is allowed to pass through the capsule 20, i.e.the capsule 20 is transmissive.

[0083] Referring now to FIG. 3B, the particles 50 are dispersed into thecapsule 20 by applying an AC field to the capsule 20 via the electrodes30, 40. The particles 50, dispersed into the capsule 20 by the AC field,block incident light from passing through the capsule 20, causing it toappear dark at the viewpoint 10. The embodiment depicted in FIGS. 3A-3Bmay be used in shutter mode by not providing the reflecting surface 60and instead providing a translucent layer, a transparent layer, or nolayer at all. In shutter mode, application of an AC electric fieldcauses the capsule 20 to appear opaque. The transparency of a shuttermode display formed by the apparatus depicted in FIGS. 3A-3D may becontrolled by the number of capsules addressed using DC fields and ACfields. For example, a display in which every other capsule 20 isaddressed using an AC field would appear fifty percent transmissive.

[0084]FIGS. 3C and 3D depict an embodiment of the electrode structuredescribed above in which electrodes 30, 40 are on “top” of the capsule20, that is, the electrodes 30, 40 are between the viewpoint 10 and thecapsule 20. In these embodiments, both electrodes 30, 40 should betransparent. Transparent polymers can be fabricated using conductivepolymers, such as polyaniline, polythiophenes, or indium tin oxide.These materials may be made soluble so that electrodes can be fabricatedusing coating techniques such as spin coating, spray coating, meniscuscoating, printing techniques, forward and reverse roll coating and thelike. In these embodiments, light passes through the electrodes 30, 40and is either absorbed by the particles 50, reflected by retroreflectinglayer 60 (when provided), or transmitted throughout the capsule 20 (whenretroreflecting layer 60 is not provided).

[0085] The addressing structure depicted in FIGS. 3A-3D may be used withelectrophoretic display media and encapsulated electrophoretic displaymedia. FIGS. 3A-3D depict embodiments in which electrode 30, 40 arestatically attached to the display media. In certain embodiments, theparticles 50 exhibit bistability, that is, they are substantiallymotionless in the absence of a electric field. In these embodiments, theelectrodes 30, 40 may be provided as part of a “stylus” or other devicewhich is scanned over the material to address each capsule or cluster ofcapsules. This mode of addressing particle-based displays will bedescribed in more detail below in connection with FIG. 16.

[0086] Referring now to FIGS. 4A and 4B, a capsule 20 of aelectronically addressable media is illustrated in which the techniqueillustrated above is used with multiple rear-addressing electrodes. Thecapsule 20 contains at least one particle 50 dispersed in a clearsuspending fluid 25. The capsule 20 is addressed by multiple smallerelectrodes 30 and multiple larger electrodes 40. In these embodiments,the smaller electrodes 30 may be selected to collectively address equalor less than one-half the area the larger electrodes 40 addresses, inother words, no more than half the “size” of the electrodes 40. Infurther embodiments, the smaller electrodes 30 are collectivelyone-fourth the “size” of the larger electrodes 40. In furtherembodiments the smaller electrodes 30 are collectively one-eighth the“size” of the larger electrodes 40. In preferred embodiments, thesmaller electrodes 30 are collectively one-sixteenth the “size” of thelarger electrodes.

[0087] Each electrode 30 may be provided as separate electrodes that arecontrolled in parallel to control the display. For example, eachseparate electrode may be substantially simultaneously set to the samevoltage as all other electrodes of that size. Alternatively, theelectrodes 30, 40 may be interdigitated to provide the embodiment shownin FIGS. 4A and 4B.

[0088] Operation of the rear-addressing electrode structure depicted inFIGS. 4A and 4B is similar to that described above. For example, thecapsule 20 may contain positively charged, black particles 50 dispersedin a substantially clear suspending fluid 25. The smaller electrodes 30are colored black and the larger electrodes 40 are colored white or arehighly reflective. Referring to FIG. 4A, the smaller electrodes 30 areplaced at a negative potential relative to the larger electrodes 40,causing particles 50 migrate within the capsule to the smallerelectrodes 30 and the capsule 20 appears to the viewpoint 10 as a mix ofthe larger, white electrodes 40 and the smaller, black electrodes 30,creating an effect which is largely white.

[0089] Referring to FIG. 4B, when the smaller electrodes 30 are placedat a positive potential relative to the larger electrodes 40, particles50 migrate to the larger electrodes 40 causing the capsule 20 to displaya mix of the larger, white electrodes 40 occluded by the black particles50 and the smaller, black electrodes 30, creating an effect which islargely black. The techniques described above with respect to theembodiments depicted in FIGS. 1A and 1B for producing two-color displayswork with equal effectiveness in connection with these embodiments.

[0090]FIGS. 5A and 5B depict an embodiment of a rear-addressingelectrode structure that creates a reflective color display in a mannersimilar to halftoning or pointillism. The capsule 20 contains whiteparticles 55 dispersed in a clear suspending fluid 25. Electrodes 42,44, 46, 48 are colored cyan, magenta, yellow, and white respectively.Referring to FIG. 5A, when the colored electrodes 42, 44, 46 are placedat a positive potential relative to the white electrode 48,negatively-charged particles 55 migrate to these three electrodes,causing the capsule 20 to present to the viewpoint 10 a mix of the whiteparticles 55 and the white electrode 48, creating an effect which islargely white. Referring to FIG. 5B, when electrodes 42, 44, 46 areplaced at a negative potential relative to electrode 48, particles 55migrate to the white electrode 48, and the eye 10 sees a mix of thewhite particles 55, the cyan electrode 42, the magenta electrode 44, andthe yellow electrode 46, creating an effect which is largely black orgray.

[0091] By addressing the electrodes, any color can be produced that ispossible with a subtractive color process. For example, to cause thecapsule 20 to display an orange color to the viewpoint 10, the yellowelectrode 46 and the magenta electrode 42 are set to a voltage potentialthat is more positive than the voltage potential applied by the cyanelectrode 42 and the white electrode 48. Further, the relativeintensities of these colors can be controlled by the actual voltagepotentials applied to the electrodes.

[0092] In another embodiment, depicted in FIG. 6, a color display isprovided by a capsule 20 of size d containing multiple species ofparticles in a clear, dispersing fluid 25. Each species of particles hasdifferent optical properties and possess different electrophoreticmobilities (μ) from the other species. In the embodiment depicted inFIG. 6, the capsule 20 contains red particles 52, blue particles 54, andgreen particles 56, and

|μ_(R)|>|μ_(B)|>|μ_(G)|

[0093] That is, the magnitude of the electrophoretic mobility of the redparticles 52, on average, exceeds the electrophoretic mobility of theblue particles 54, on average, and the electrophoretic mobility of theblue particles 54, on average, exceeds the average electrophoreticmobility of the green particles 56. As an example, there may be aspecies of red particle with a zeta potential of 100 millivolts (mV), ablue particle with a zeta potential of 60 mV, and a green particle witha zeta potential of 20 mV. The capsule 20 is placed between twoelectrodes 32, 42 that apply an electric field to the capsule.

[0094] FIGS. 7A-7B depict the steps to be taken to address the displayshown in FIG. 6 to display a red color to a viewpoint 10. Referring toFIG. 7A, all the particles 52, 54, 56 are attracted to one side of thecapsule 20 by applying an electric field in one direction. The electricfield should be applied to the capsule 20 long enough to attract eventhe more slowly moving green particles 56 to the electrode 34. Referringto FIG. 7B, the electric field is reversed just long enough to allow thered particles 52 to migrate towards the electrode 32. The blue particles54 and green particles 56 will also move in the reversed electric field,but they will not move as fast as the red particles 52 and thus will beobscured by the red particles 52. The amount of time for which theapplied electric field must be reversed can be determined from therelative electrophoretic mobilities of the particles, the strength ofthe applied electric field, and the size of the capsule.

[0095] FIGS. 8A-8D depict addressing the display element to a bluestate. As shown in FIG. 8A, the particles 52, 54, 56 are initiallyrandomly dispersed in the capsule 20. All the particles 52, 54, 56 areattracted to one side of the capsule 20 by applying an electric field inone direction (shown in FIG. 8B). Referring to FIG. 8C, the electricfield is reversed just long enough to allow the red particles 52 andblue particles 54 to migrate towards the electrode 32. The amount oftime for which the applied electric field must be reversed can bedetermined from the relative electrophoretic mobilities of theparticles, the strength of the applied electric field, and the size ofthe capsule. Referring to FIG. 8D, the electric field is then reversed asecond time and the red particles 52, moving faster than the blueparticles 54, leave the blue particles 54 exposed to the viewpoint 10.The amount of time for which the applied electric field must be reversedcan be determined from the relative electrophoretic mobilities of theparticles, the strength of the applied electric field, and the size ofthe capsule.

[0096] FIGS. 9A-9C depict the steps to be taken to present a greendisplay to the viewpoint 10. As shown in FIG. 9A, the particles 52, 54,56 are initially distributed randomly in the capsule 20. All theparticles 52, 54, 56 are attracted to the side of the capsule 20proximal the viewpoint 10 by applying an electric field in onedirection. The electric field should be applied to the capsule 20 longenough to attract even the more slowly moving green particles 56 to theelectrode 32. As shown in FIG. 9C, the electric field is reversed justlong enough to allow the red particles 52 and the blue particles 54 tomigrate towards the electrode 54, leaving the slowly-moving greenparticles 56 displayed to the viewpoint. The amount of time for whichthe applied electric field must be reversed can be determined from therelative electrophoretic mobilities of the particles, the strength ofthe applied electric field, and the size of the capsule.

[0097] In other embodiments, the capsule contains multiple species ofparticles and a dyed dispersing fluid that acts as one of the colors. Instill other embodiments, more than three species of particles may beprovided having additional colors. Although FIGS. 6-9C depict twoelectrodes associated with a single capsule, the electrodes may addressmultiple capsules or less than a full capsule

[0098] In FIG. 10, the rear substrate 100 for a seven segment display isshown that improves on normal rear electrode structures by providing ameans for arbitrarily connecting to any electrode section on the rear ofthe display without the need for conductive trace lines on the surfaceof the patterned substrate or a patterned counter electrode on the frontof the display. Small conductive vias through the substrate allowconnections to the rear electrode structure. On the back of thesubstrate, these vias are connected to a network of conductors. Theseconductors can be run so as to provide a simple connection to the entiredisplay. For example, segment 112 is connected by via 114 through thesubstrate 116 to conductor 118. A network of conductors may run multipleconnections (not shown) to edge connector 122. This connector can bebuilt into the structure of the conductor such as edge connector 122.Each segment of the rear electrode can be individually addressed easilythrough edge connector 122. A continuous top electrode can be used withthe substrate 116.

[0099] The rear electrode structure depicted in FIG. 10 is useful forany display media, but is particularly advantageous for particle-baseddisplays because such displays do not have a defined appearance when notaddressed. The rear electrode should be completely covered in anelectrically conducting material with room only to provide necessaryinsulation of the various electrodes. This is so that the connections onthe rear of the display can be routed with out concern for affecting theappearance of the display. Having a mostly continuous rear electrodepattern assures that the display material is shielded from the rearelectrode wire routing.

[0100] In FIG. 11, a 3×3 matrix is shown. Here, matrix segment 124 on afirst side of substrate 116 is connected by via 114 to conductor 118 ona second side of substrate 116. The conductors 18 run to an edge andterminate in a edge connector 122. Although the display element of FIG.11 shows square segments 124, the segments may be shaped or sized toform a predefined display pattern.

[0101] In FIG. 12, a printed circuit board 138 is used as the rearelectrode structure. The front of the printed circuit board 138 hascopper pads 132 etched in the desired shape. There are plated vias 114connecting these electrode pads to an etched wire structure 136 on therear of the printed circuit board 138. The wires 136 can be run to oneside or the rear of the printed circuit board 138 and a connection canbe made using a standard connector such as a surface mount connector orusing a flex connector and anisotropic glue (not shown). Vias may befilled with a conductive substance, such as solder or conductive epoxy,or an insulating substance, such as epoxy.

[0102] Alternatively, a flex circuit such a copper-clad polyimide may beused for the rear electrode structure of FIG. 10. Printed circuit board138 may be made of polyimide, which acts both as the flex connector andas the substrate for the electrode structure. Rather than copper pads132, electrodes (not shown) may be etched into the copper covering thepolyimide printed circuit board 138. The plated vias 114 connect theelectrodes etched onto the substrate the rear of the printed circuitboard 138, which may have an etched conductor network thereon (theetched conductor network is similar to the etched wire structure 136).

[0103] In FIG. 13, a thin dielectric sheet 150, such as polyester,polyimide, or glass can be used to make a rear electrode structure.Holes 152 are punched, drilled, abraded, or melted through the sheetwhere conductive paths are desired. The front electrode 154 is made ofconductive ink printed using any technique described above. The holesshould be sized and the ink should be selected to have a viscosity sothat the ink fills the holes. When the back structure 156 is printed,again using conductive ink, the holes are again filled. By this method,the connection between the front and back of the substrate is madeautomatically.

[0104] In FIG. 14, the rear electrode structure can be made entirely ofprinted layers. A conductive layer 166 can be printed onto the back of adisplay comprised of a clear, front electrode 168 and a printabledisplay material 170. A clear electrode may be fabricated from indiumtin oxide or conductive polymers such as polyanilines andpolythiophenes. A dielectric coating 176 can be printed leaving areasfor vias. Then, the back layer of conductive ink 178 can be printed. Ifnecessary, an additional layer of conductive ink can be used before thefinal ink structure is printed to fill in the holes.

[0105] This technique for printing displays can be used to build therear electrode structure on a display or to construct two separatelayers that are laminated together to form the display. For example anelectronically active ink may be printed on an indium tin oxideelectrode. Separately, a rear electrode structure as described above canbe printed on a suitable substrate, such as plastic, polymer films, orglass. The electrode structure and the display element can be laminatedto form a display.

[0106] Referring now to FIG. 15, a threshold may be introduced into anelectrophoretic display cell by the introduction of a third electrode.One side of the cell is a continuous, transparent electrode 200 (anode).On the other side of the cell, the transparent electrode is patternedinto a set of isolated column electrode strips 210. An insulator 212covers the column electrodes 210, and an electrode layer on top of theinsulator is divided into a set of isolated row electrode strips 230,which are oriented orthogonal to the column electrodes 210. The rowelectrodes 230 are patterned into a dense array of holes, or a grid,beneath which the exposed insulator 212 has been removed, forming amultiplicity of physical and potential wells.

[0107] A positively charged particle 50 is loaded into the potentialwells by applying a positive potential (e.g. 30V) to all the columnelectrodes 210 while keeping the row electrodes 230 at a less positivepotential (e.g. 15V) and the anode 200 at zero volts. The particle 50may be a conformable capsule that situates itself into the physicalwells of the control grid. The control grid itself may have arectangular cross-section, or the grid structure may be triangular inprofile. It can also be a different shape which encourages themicrocapsules to situate in the grid, for example, hemispherical.

[0108] The anode 200 is then reset to a positive potential (e.g. 50V).The particle will remain in the potential wells due to the potentialdifference in the potential wells: this is called the Hold condition. Toaddress a display element the potential on the column electrodeassociated with that element is reduced, e.g. by a factor of two, andthe potential on the row electrode associated with that element is madeequal to or greater than the potential on the column electrode. Theparticles in this element will then be transported by the electric fielddue to the positive voltage on the anode 200. The potential differencebetween row and column electrodes for the remaining display elements isnow less than half of that in the normal Hold condition.

[0109] The geometry of the potential well structure and voltage levelsare chosen such that this also constitutes a Hold condition, i.e., noparticles will leave these other display elements and hence there willbe no half-select problems. This addressing method can select and writeany desired element in a matrix without affecting the pigment in anyother display element. A control electrode device can be operated suchthat the anode electrode side of the cell is viewed.

[0110] The control grid may be manufactured through any of the processesknown in the art, or by several novel processes described herein. Thatis, according to traditional practices, the control grid may beconstructed with one or more steps of photolithography and subsequentetching, or the control grid may be fabricated with a mask and a“sandblasting” technique.

[0111] In another embodiment, the control grid is fabricated by anembossing technique on a plastic substrate. The grid electrodes may bedeposited by vacuum deposition or sputtering, either before or after theembossing step. In another embodiment, the electrodes are printed ontothe grid structure after it is formed, the electrodes consisting of somekind of printable conductive material which need not be clear (e.g. ametal or carbon-doped polymer, an intrinsically conducting polymer,etc.).

[0112] In a preferred embodiment, the control grid is fabricated with aseries of printing steps. The grid structure is built up in a series ofone or more printed layers after the cathode has been deposited, and thegrid electrode is printed onto the grid structure. There may beadditional insulator on top of the grid electrode, and there may bemultiple grid electrodes separated by insulator in the grid structure.The grid electrode may not occupy the entire width of the gridstructure, and may only occupy a central region of the structure, inorder to stay within reproducible tolerances. In another embodiment, thecontrol grid is fabricated by photoetching away a glass, such as aphotostructural glass.

[0113] In an encapsulated electrophoretic image display, anelectrophoretic suspension, such as the ones described previously, isplaced inside discrete compartments that are dispersed in a polymermatrix. This resulting material is highly susceptible to an electricfield across the thickness of the film. Such a field is normally appliedusing electrodes attached to either side of the material. However, asdescribed above in connection with FIGS. 3A-3D, some display media maybe addressed by writing electrostatic charge onto one side of thedisplay material. The other side normally has a clear or opaqueelectrode. For example, a sheet of encapsulated electrophoretic displaymedia can be addressed with a head providing DC voltages.

[0114] In another implementation, the encapsulated electrophoreticsuspension can be printed onto an area of a conductive material such asa printed silver or graphite ink, aluminized mylar, or any otherconductive surface. This surface which constitutes one electrode of thedisplay can be set at ground or high voltage. An electrostatic headconsisting of many electrodes can be passed over the capsules toaddressing them. Alternatively, a stylus can be used to address theencapsulated electrophoretic suspension.

[0115] In another implementation, an electrostatic write head is passedover the surface of the material. This allows very high resolutionaddressing. Since encapsulated electrophoretic material can be placed onplastic, it is flexible. This allows the material to be passed throughnormal paper handling equipment. Such a system works much like aphotocopier, but with no consumables. The sheet of display materialpasses through the machine and an electrostatic or electrophotographichead addresses the sheet of material.

[0116] In another implementation, electrical charge is built up on thesurface of the encapsulated display material or on a dielectric sheetthrough frictional or triboelectric charging. The charge can built upusing an electrode that is later removed. In another implementation,charge is built up on the surface of the encapsulated display by using asheet of piezoelectric material.

[0117]FIG. 16 shows an electrostatically written display. Stylus 300 isconnected to a positive or negative voltage. The head of the stylus 300can be insulated to protect the user. Dielectric layer 302 can be, forexample, a dielectric coating or a film of polymer. In otherembodiments, dielectric layer 302 is not provided and the stylus 300contacts the encapsulated electrophoretic display 304 directly.Substrate 306 is coated with a clear conductive coating such as ITOcoated polyester. The conductive coating is connected to ground. Thedisplay 304 may be viewed from either side.

[0118] Microencapsulated displays offer a useful means of creatingelectronic displays, many of which can be coated or printed. There aremany versions of microencapsulated displays, including microencapsulatedelectrophoretic displays. These displays can be made to be highlyreflective, bistable, and low power.

[0119] To obtain high resolution displays, it is useful to use someexternal addressing means with the microencapsulated material. Thisinvention describes useful combinations of addressing means withmicroencapsulated electrophoretic materials in order to obtain highresolution displays.

[0120] One method of addressing liquid crystal displays is the use ofsilicon-based thin film transistors to form an addressing backplane forthe liquid crystal. For liquid crystal displays, these thin filmtransistors are typically deposited on glass, and are typically madefrom amorphous silicon or polysilicon. Other electronic circuits (suchas drive electronics or logic) are sometimes integrated into theperiphery of the display. An emerging field is the deposition ofamorphous or polysilicon devices onto flexible substrates such as metalfoils or plastic films.

[0121] The addressing electronic backplane could incorporate diodes asthe nonlinear element, rather than transistors. Diode-based activematrix arrays have been demonstrated as being compatible with liquidcrystal displays to form high resolution devices.

[0122] There are also examples of crystalline silicon transistors beingused on glass substrates. Crystalline silicon possesses very highmobilities, and thus can be used to make high performance devices.Presently, the most straightforward way of constructing crystallinesilicon devices is on a silicon wafer. For use in many types of liquidcrystal displays, the crystalline silicon circuit is constructed on asilicon wafer, and then transferred to a glass substrate by a “liftoff”process. Alternatively, the silicon transistors can be formed on asilicon wafer, removed via a liftoff process, and then deposited on aflexible substrate such as plastic, metal foil, or paper. As anotherimplementation, the silicon could be formed on a different substratethat is able to tolerate high temperatures (such as glass or metalfoils), lifted off, and transferred to a flexible substrate. As yetanother implementation, the silicon transistors are formed on a siliconwafer, which is then used in whole or in part as one of the substratesfor the display.

[0123] The use of silicon-based circuits with liquid crystals is thebasis of a large industry. Nevertheless, these display possess seriousdrawbacks. Liquid crystal displays are inefficient with light, so thatmost liquid crystal displays require some sort of backlighting.Reflective liquid crystal displays can be constructed, but are typicallyvery dim, due to the presence of polarizers. Most liquid crystal devicesrequire precise spacing of the cell gap, so that they are not verycompatible with flexible substrates. Most liquid crystal displaysrequire a “rubbing” process to align the liquid crystals, which is bothdifficult to control and has the potential for damaging the TFT array.

[0124] The combination of these thin film transistors withmicroencapsulated electrophoretic displays should be even moreadvantageous than with liquid crystal displays. Thin film transistorarrays similar to those used with liquid crystals could also be usedwith the microencapsulated display medium. As noted above, liquidcrystal arrays typically requires a “rubbing” process to align theliquid crystals, which can cause either mechanical or static electricaldamage to the transistor array. No such rubbing is needed formicroencapsulated displays, improving yields and simplifying theconstruction process.

[0125] Microencapsulated electrophoretic displays can be highlyreflective. This provides an advantage in high-resolution displays, as abacklight is not required for good visibility. Also, a high-resolutiondisplay can be built on opaque substrates, which opens up a range of newmaterials for the deposition of thin film transistor arrays.

[0126] Moreover, the encapsulated electrophoretic display is highlycompatible with flexible substrates. This enables high-resolution TFTdisplays in which the transistors are deposited on flexible substrateslike flexible glass, plastics, or metal foils. The flexible substrateused with any type of thin film transistor or other nonlinear elementneed not be a single sheet of glass, plastic, metal foil, though.Instead, it could be constructed of paper. Alternatively, it could beconstructed of a woven material. Alternatively, it could be a compositeor layered combination of these materials.

[0127] As in liquid crystal displays, external logic or drive circuitrycan be built on the same substrate as the thin film transistor switches.

[0128] In another implementation, the addressing electronic backplanecould incorporate diodes as the nonlinear element, rather thantransistors.

[0129] In another implementation, it is possible to form transistors ona silicon wafer, dice the transistors, and place them in a large areaarray to form a large, TFT-addressed display medium. One example of thisconcept is to form mechanical impressions in the receiving substrate,and then cover the substrate with a slurry or other form of thetransistors. With agitation, the transistors will fall into theimpressions, where they can be bonded and incorporated into the devicecircuitry. The receiving substrate could be glass, plastic, or othernonconductive material. In this way, the economy of creating transistorsusing standard processing methods can be used to create large-areadisplays without the need for large area silicon processing equipment.

[0130] While the examples described here are listed using encapsulatedelectrophoretic displays, there are other particle-based display mediawhich should also work as well, including encapsulated suspendedparticles and rotating ball displays.

[0131] Now referring to FIGS. 17-20, some display media include portionsor components that do not contribute to the changing appearance ofimages displayed during operation of a display device. In particular,portions of a displayed image can have a fixed optical appearance. Thiseffect can be described with reference to an illustrative display mediumdepicted in FIGS. 17A and 17B.

[0132]FIG. 17A is a cross-sectional view of an embodiment of anelectrophoretic display medium 400. The medium 400 includeselectrophoretic material 410, for example, the above-describedelectrophoretic phase, and binder 420. The electrophoretic material maybe directly encapsulated within voids in the binder 420 or reside withincapsule membranes embedded in the binder 420.

[0133] The electrophoretic material 410 in the embodiment illustrated inFIG. 17A includes a suspending fluid and at least one electrophoreticparticle. The suspending fluid has an optical property, and may be clearor dyed. The one or more particles may have an optical property that isdifferent from that of the suspending fluid. The particles may includemore than one type of particle. Different particle types may havedifferent optical properties, different electrophoretic responses, andmay be included in the same or different capsules. Further detailsregarding electrophoretic display materials are described at the endthis Detailed Description of the Invention.

[0134]FIG. 17B is a planar, two-dimensional projected view of theembodiment of the display medium 400, corresponding to FIG. 17A. Asillustrated by FIG. 17B, a portion of the display surface has a fixedoptical appearance, as seen by a user of the display. This portion ofthe display corresponds to regions that include only binder 420 in theviewed two-dimensional projection of the electrophoretic display medium400. In contrast, viewed portions of the display that includeelectrophoretic material 410 can produce changing opticalcharacteristics, for example, changing colors or reflectance. Thedisplay can thus present images to a user, though the images include afixed portion, i.e., a fixed background.

[0135] More generally, binder, capsule membranes and other materialswith a fixed optical state, and which extend through the viewedthickness of a display medium, may contribute to a fixed portion of animage presented by a display. The fixed optical state portion of thedisplay typically dilutes the image quality produced by the variableoptical state portion.

[0136] The optically fixed portions of a display may be transparent,translucent or opaque. The fixed portions may have an optical propertythat is predetermined, that is, selected during design or manufacturingof the display. The characteristic may be modified by, for example,changing a characteristic of the optically fixed components of a displayor adding additional structures, such as additional layers, to thedisplay structure. The predetermined property may be, for example, areflectance, a transmittance, a brightness or a color.

[0137] The optically fixed components of a display that can be selectedto have a particular characteristic are any of the components that canbe observed, at least in part, by a viewer of the display. Theseinclude, for example, a top (light transmitting) electrode, a bindermaterial, or a bottom electrode seen through the binder. The materialsfrom which these structures are formed can be selected for its opticalproperties. Alternatively, materials, for example, in atomic, molecularor particulate form, may be added to a structure to modify its opticalproperties.

[0138] Added structures can include, for example, a layer positioned atvarious levels of a display element sandwich. In the followingdescription, various added structures, materials added to existingstructures, or modified existing structures, are referred to as “opticalbiasing elements”. These materials and structures may also be referredto as “background” components because they contribute to the opticallyfixed portion of a display.

[0139] If the fixed portion of a display is not completely black, thefixed portion will limit the degree of black (i.e. the dark state) thatthe display can present. If the fixed portion is not completely white,the degree of white will be limited. Thus, the contrast range of thedisplay may also be limited by the fixed portion.

[0140] Typically, the reflectance and contrast ratio of a display arethe optical properties of most interest. For example, the whitereflectance is a ratio of the white state of the display to a whitestate of a standard, the standard representing 100% reflectance. Thecontrast ratio is generally defined as the ratio of the whitereflectance to the dark reflectance. These properties control thevisibility and legibility of a display. Though ideal values of theoptical characteristics vary for different applications, one generallyprefers a white reflectance and a contrast ratio that are as high aspossible.

[0141] When designing an electrophoretic display element, one can maketradeoffs in the selection of optical characteristics. For example,increases in the dye concentration of a fluid, in a suspending fluid andparticle-based electrophoretic material, can serve to reduce both thewhite and dark reflectance of the variable portions of a display. Anincrease in the particle concentration in such an electrophoreticmaterial can increase both the white and dark reflectances. Opticalproperties can also be changed by changing the thickness of theelectrophoretic display medium 400, for example, by reducing the capsulesize.

[0142] The white reflectance, dark reflectance and contrast ratio of adisplay can be estimated with the below equations. The white reflectanceof a display may be expressed as:

R _(W)=(R _(MC−W))(A _(S))+(R _(BKGND))(1−A _(S))   Eq. 1

[0143] The dark/black reflectance as:

R _(D)=(R _(MC−D))(A _(S))+(R _(BKCND))(1−A _(S))   Eq. 2

[0144] The contrast ratio as: $\begin{matrix}{C_{R} = {\frac{R_{W}}{R_{D}} = \frac{{\left( R_{{MC} - W} \right)\left( A_{S} \right)} + {\left( R_{BKGND} \right)\left( {1 - A_{S}} \right)}}{{\left( R_{{MC} - D} \right)\left( A_{S} \right)} + {\left( R_{BKGND} \right)\left( {1 - A_{S}} \right)}}}} & {{Eq}.\quad 3}\end{matrix}$

[0145] which may be expressed as: $\begin{matrix}{C_{R} = {\frac{R_{W}}{R_{D}} = \frac{\left( R_{{MC} - W} \right) + {\left( R_{BKGND} \right){\left( {1 - A_{S}} \right)/\left( A_{S} \right)}}}{\left( R_{{MC} - D} \right) + {\left( R_{BKGND} \right){\left( {1 - A_{S}} \right)/\left( A_{S} \right)}}}}} & {{Eq}.\quad 4}\end{matrix}$

[0146] where:

[0147] R_(W) is the reflectance (in percent) of a display having a whiteappearance, as a percentage of the reflectance of a standard whitematerial that is taken as having 100% reflectance;

[0148] R_(D) is the reflectance (in percent) of a display having a blackappearance, as a percentage of the reflectance of a standard whitematerial that is taken as having 100% reflectance;

[0149] R_(MC−W) is the reflectance (in percent) of a unit area ofcapsules having a white appearance, as a percentage of the reflectanceof a standard white material that is taken as having 100% reflectance;

[0150] R_(MC−D) is the reflectance (in percent) of a unit area ofcapsules having a black appearance, as a percentage of the reflectanceof a standard white material that is taken as having 100% reflectance;

[0151] R_(BKGND) is the reflectance (in percent) of a unit area of thebackground of the display, as a percentage of the reflectance of astandard white material that is taken as having 100% reflectance;

[0152] A_(SWITCHABLE) or A_(S) is the area, as a percentage of the totalsurface area of the display, of the portion of a display that can be setto display a first optical property or a second optical property, forexample white and black. Hence, the value (1−A_(SWITCHABLE)) or(1−A_(S)) denotes the percentage of the total area of the display thatcannot be changed in appearance, i.e., the fixed viewable portion of thedisplay; and

[0153] C_(R) is the contrast ratio, or R_(W)/R_(D).

[0154] Equation 4, for example, illustrates the effect on the contrastratio of changes in the various reflectances when the area of thecapsules as a percentage of the total display area, or equivalently, theratio (1−A_(S))/(A_(S)), is held constant. One can also note the effecton the contrast ratio of changes in the area of the capsules as apercentage of the total display area when the various reflectances areheld constant.

[0155] If, for example, A_(S), R_(MC−W) and R_(MC−D) are held constant,with R_(MC−W)>R_(MC−D), an increase in R_(BKGND) will decrease thecontrast ratio C_(R), while a decrease in R_(BKGND) will increase C_(R).This result is obtained because the addition of a fixed amount ofreflectance through an increase in R_(BKGND) is a smaller increaseproportionately to the larger quantity R_(MC−W) than it is to thesmaller quantity R_(MC−D). Increasing R_(BKGND), however, will increasethe overall reflectance of either display state, causing the display toappear brighter, with a smaller contrast ratio. Conversely, a decreaseof R_(BKGND) will cause the display to appear darker, with greatercontrast ratio.

[0156] Alternatively, the total reflectance, as well as the spectraldistribution of the reflectance, of the background may be changed, forexample, by changing the color or tone of the background. This wouldalter the appearance of the display with regard to one or more of thebrightness, the contrast and the color or tone of the various displaystates. The illustrative embodiment of FIGS. 17A and 17B has beenpresented with regard to a display that comprises capsules that offeronly a black appearance and a white appearance. As described elsewherein this Detailed Description, other embodiments, such as displays havingcapsules comprising multiple colored particles, colored suspendingfluids and colored electrodes can be provided according to theprinciples of the invention. The use of a biasing element as describedherein with such other embodiments can affect many of the opticalproperties of such displays.

[0157] The above-shown model equations illustrate the potential tocontrol the optical characteristics of a display by selecting theoptical characteristics of an optically fixed portion of the display. Anoptical biasing element can be added to, for example, an electrophoreticdisplay element to achieve this control of the fixed portion of thedisplay element. An optical biasing element can include variousmaterials, and have various locations in the display element structure,as illustrated in the embodiments described with reference to FIGS.18-20.

[0158] Various embodiments of a display element that include an opticalbiasing layer are described with reference to FIGS. 18A, 18B and 18C.FIG. 18A is a cross-sectional view of a display element 500A thatincludes a top substrate 462 and a bottom substrate 461. The bottomsubstrate 461 may include, for example, a rigid layer, such as glass, ora flexible sheet, such as polyimide. The bottom substrate 461 may bepart of an electrical backplane of a display.

[0159] The display element 500A includes an electrophoretic displaymedium 400, which includes electrophoretic material 410 and binder 420.The electrophoretic material may be directly encapsulated within voidsin the binder 420 or reside within capsule membranes embedded in thebinder 420. The display element 500A also includes, and is addressedvia, a top electrode 440 and a bottom electrode 450. The top electrodeand the top substrate are light transmissive to permit observation ofthe electrophoretic display medium 400.

[0160] The display element 500A includes an optical biasing element 450,located between the electrophoretic display medium 400 and the bottomelectrode 430. In the present embodiment, the optical biasing element450 is a sheet or layer of material that is selected for its opticalcharacteristics, as described above.

[0161] The optical biasing element 450 may be fabricated, for example,by depositing, coating, printing or laminating material adjacent to thebottom electrode or the bottom substrate. A biasing element may includea thermoplastic sheet or an adhesive layer that may help to laminatelayers of a display element.

[0162] As illustrated in FIG. 18B, a display element 500B may include anoptical biasing element 430 that is located between the bottom substrate461 and the bottom electrode 430. As illustrated in FIG. 18C, a displayelement 500C may include an optical biasing element 430 located betweenthe top substrate 462 and the electrophoretic display medium 400.

[0163] Other embodiments include an optical biasing element at otherlocations, with the requirement that at least a portion of the opticalbiasing element be viewable by an observer of the display. Further, theoptical biasing element should be sufficiently transmissive to light topermit viewing of the electrophoretic material, if the optical biasingelement overlays the electrophoretic material. If the biasing elementlies beneath the electrophoretic display medium 400, the element may betransmissive or opaque.

[0164] Referring to FIGS. 19 and 20, other embodiments incorporate anoptical biasing element within other display element components, ratherthan as a separate layer or other distinctly separate structure. FIGS.19A and 19B are cross-sectional views of embodiments that incorporate,or embed, an optical biasing element in a binder material.

[0165]FIG. 19A illustrates an embodiment of a display element 500D withan electrophoretic display medium layer that includes binder 420.Embedded in the binder is an optical biasing element that includesparticles 451. During manufacturing, for example, particles 451 may beadded to a binder in its liquid state, prior to mixing with anencapsulated electrophoretic material.

[0166] The particles 451 are selected for their optical properties. Theparticles 451 may include, for example, carbon black or pigment. Thepigment may be, for example, white pigments such as titanium dioxide,barium sulfate and barium titanate. The particles may include one ormore metals, for example, noble metals such as silver, gold andpalladium.

[0167]FIG. 19B illustrates an embodiment of a display element 500E withan electrophoretic display medium layer that includes an opticallymodified binder 425. Embedded in the binder is an optical biasingelement that includes atoms or molecules. The optical biasing element isselected for its ability to modify an optical characteristic of thebinder.

[0168]FIGS. 20A and 20B illustrate embodiments in which an opticalbiasing element is incorporated into an electrode layer. FIG. 20a is across-sectional view of an embodiment of a display element 500F thatincludes a bottom electrode 430. An optical biasing element is embeddedin the bottom electrode 430. The optical biasing element includesparticles 451A that are selected for an optical characteristic.

[0169] The particles may include, for example, pigment particles such astitanium dioxide, barium sulfate and barium titanate particles. Theparticles may be incorporated into an electrode formed from a polymericmaterial. For example, carbon-containing particles may be incorporatedin a polymeric carrier to produce a black appearing electrode.

[0170]FIG. 20B is a cross section of an embodiment of a display element500G. A molecular, or atomic optical biasing element is incorporatedinto a bottom electrode to provide an optically modified electrode 435.

[0171] In other embodiments, an optical biasing element is coated on topof an electrode. For example, metallic particles such as silver, gold orpalladium may be coated on the electrode. Alternatively, an electrodematerial may be selected to provide both necessary conductivity for theelectrode and to provide an optical characteristic. Thus, the electrodemay be both an electrode and an optical biasing element. Similarly,other components of a display element may be formed of materials toenable such a dual function.

[0172] Now referring to FIGS. 21-25, according to principles of theinvention, some displays include one or more layers that can enhance thebrightness of display images. Throughout the views of FIGS. 21-25,arrows represent various rays of light.

[0173] In particular, a refractive layer or film with a relatively lowerrefractive index (the “low-index refractive layer”), in combination witha material portion having a higher index of refraction and disposedbetween reflective particles of the display medium and the low-indexrefractive layer, can reduce the loss of light associated with internalreflection. The material portion having a higher index of refraction caninclude a portion of an encapsulated display medium, such as a capsulemembrane, a cell structure or a binder material. In embodiments ofnon-encapsulated displays, the material portion with a higher refractiveindex can include a barrier structure, or separation structure enclosinga display medium (e.g. a fluid suspended with particles). Generalprinciples of light loss associated with internal reflection are nextdescribed with reference to an illustrative display depicted in FIG. 21.

[0174] The term “pixel,” as used herein, refers to a portion of thedisplay medium that includes, for example, a portion of one cell or onecapsule, a complete cell or capsule, or more than one cell or capsule. Apixel can be defined, for example, by addressing electrodes positionedadjacent to the bottom surface of the display medium.

[0175] The term “optical stack,” as used herein, refers to the materialsthrough which light must pass to scatter from the particles (and throughwhich scattered light must pass to be observed by the display user).

[0176]FIG. 21 shows a cross-sectional view of an embodiment of anelectrophoretic display 600. The display includes an electrophoreticdisplay medium 610, which includes capsules 612, electrophoreticparticles 614, and binder 616. The display further includes a frontlayer 620 (e.g., a window or window layer) that can be in contact with atransparent conductor 622. The window 620 can be, for example, glass orplastic. The display can further include backplane electronics 630,which include a substrate 632 and an electrode 634.

[0177] Incident light from the ambient environment 650 (e.g. air)following, for example, path X is refracted when it passes through thefront ambient environment-window interface 660. The light passes intothe electrophoretic display medium 610, and undergoes multiplescattering off, for example, particles 614 inside the capsules 612.

[0178] The light reflects off the particles in a Lambertian (or nearLambertian) distribution; that is, the intensity I of light varies withθ, the angle from the normal, according to approximately:

I(θ)=I(0)cos θ,

[0179] where I(0) is the intensity of light reflecting normal to thesurface of the display. Light rays that return close to the normal tothe display, at an angle from the normal θ<θ_(c), where θ_(c) is thecritical angle of the ambient environment-window interface, returnthrough the capsule, the binder, the front window, and finally out ofthe display to the observer's eye, e.g., light path Y.

[0180] The light that reflects from the particles at an angle largerthan θ_(c), however, is reflected internally, e.g., along path Z, as thelight attempts to enter a matter of a lower refractive index, forexample, at the interface between the window 620 and the ambientenvironment 650. The interface between the window 620 and the ambientenvironment 650 is sometimes referred to as the air-window interface.For monochrome displays (with and without grayscale capability), thislight has a chance at being recycled only if it impinges on aneighboring pixel that is switched to a light state (e.g., white or alight shade of gray). If the internally reflected light impinges on apixel showing a dark state, it is absorbed. Depending upon theproperties of the optical stack, optical losses due to this mechanismmay reduce display brightness by as much as 50% or more.

[0181] For a color filter based display, light that undergoes internalreflection off the air-window interface typically also has a smallerprobability of being effectively recycled. For example, for a colordisplay architecture that includes red, green, and blue filters,incident light is filtered before the first opportunity for internalreflection. Internally reflected light that originates from, forexample, a red pixel has a chance of meaningful recycling only if itreturns to a pixel that is red or reflects light close to red on thespectrum, on its second pass at being reflected off a reflectiveparticle. If red light internally reflects back to a green or blueregion, the light will be strongly absorbed and any potential recyclingeffect is effectively negated.

[0182] To reduce light loss, as described below in more detail,preferred embodiments include the low-index refractive film or layer ina microencapsulated electrophoretic display optical stack (the terms“film” and “layer” are herein used interchangeably). The low-indexrefractive layer and a material portion (in these preferred embodiments,a portion of an encapsulated display medium) together reduce lightlosses associated with the internal reflection problem. In oneembodiment, the low-index refractive layer is disposed between theencapsulated electrophoretic display medium and the front window of adisplay.

[0183] Referring to FIGS. 22A-22C, various embodiments of some upperlayers (or an optical stack portion) of displays that include reflectiveparticles and the low-index refractive layer are now described. FIG. 22Ais a cross-sectional view of an embodiment of the upper layers of adisplay 700A. The display 700A includes a front window 710, a low-indexrefractive layer 720, and can include a capping layer 730 and atransparent conductor 740. The transparent conductor 740 may be adjacentto a reflective display medium layer (embodiments of a reflectivedisplay medium layer are shown in FIGS. 23 and 25).

[0184] In some implementations, the window 710 is made of glass orplastic. The window 710 preferably has a thickness in a range from about400 to about 1100 μm for glass and from about 50 to about 500 μm forplastic. The window 710 can also include, for example, ultra-thin glassmaterial having a thickness from about 10 μm to about 300 μm.

[0185]FIG. 22B is a cross-sectional view of another embodiment of someupper layers of a display 700B. The display 700B includes the frontwindow 710, the transparent conductor 740 and the low-index refractivelayer 720. The low-index refractive layer 720 preferably has a thicknessin a range of approximately 200 nm to approximately 100 μm.

[0186]FIG. 22C is a cross-sectional view of another embodiment of theupper layers of a display 700C. The display 700C includes the frontwindow 710, the low-index refractive layer 720, the capping layer 730,color filter resist materials (for example, red material 751, greenmaterial 752 and blue material 753), and the transparent conductor 740.It should be recognized that other colors of color filter resistmaterials such as, for example, cyan, magenta and/or yellow may be used.

[0187] The low-index refractive layer 720 may have a refractive indexthat is lower than the refractive index of the window 710 (a typicalrefractive index value for the window 710 is 1.52, and a typicalrefractive index value for the low-index refractive layer 720 may be,for example, 1.22). The low-index refractive layer 720 can be chosenfrom any material with suitably low index of refraction, but it ispreferable for the index of refraction to be as close as possible tothat of the ambient environment (e.g., air, argon) that normallysurrounds the display.

[0188] Aerogels and other nanoporous materials (e.g., materials havingpores of approximately 5 nm size or less) are especially useful aslow-index refractive layer 720 where the ambient environment has anindex of refraction close to that of air (i.e. ˜1.0), since their indexof refraction typically ranges from 1.01 to 1.10. Low-index refractivelayer 720 can also include foams or other highly porous structures, andcomposites that include at least one low index material.

[0189] Some examples of materials useful for low-index refractive layerinclude nanoporous silica coatings such as NANOGLASS (HoneywellCorporation, Sunnyvale, Calif.) with an index of refraction ranging fromroughly 1.1 to 1.3, various other spin-on-glass materials such asACCUGLASS (Honeywell Corporation, Sunnyvale, Calif.) with an index ofrefraction ranging from roughly 1.2-1.5 (typically employed insemiconductor wafer processing), sodium aluminum fluoride (e.g.cryolite) with an index of refraction of 1.33 (typically employed as ananti-reflective coating for lenses). Other useful materials will beapparent to one having skill in the art. A low-index composite film, forexample, a multi-layer stack built using more than one optical material,can also be used, according to principles of the invention.

[0190] These materials can be composed of, for example, silica, alumina,aluminosilicate, graphite, carbon, ruthenium dioxide, colloidal gold,metal-oxide doped silica and alumina, niobia, titania, metal-dopedcarbon, vanadium pentoxide, zirconia, or other materials known to thoseskilled in the art.

[0191] In various embodiments, the transmission spectra of a low-indexrefractive layer 720 including a nanoporous material can be selectedsuch that the material substantially absorbs select wavelengths ofelectromagnetic radiation. For example, the low-index refractive layer720 can be designed to absorb in the UV wavelengths (for example,wavelengths less than about 390 nm) to, for example, protect theelectro-optic material from UV damage.

[0192] In further embodiments, the nanoporous material can be designedor processed in such a way that the material exhibits photoluminesence(i.e., absorbed radiative energy is emitted in the visible wavelengths).For example, the material can be doped with semiconducting, conducting,or dielectric nanoparticles or exposed to other agents during processingto impart specific photoluminesent properties to the material. Thephotoluminescence of the low-index refractive layer 720 can be used toenhance the optics of the display in particular lighting environments.

[0193] The low-index refractive layer 720 can also consist of a vacuumor gas-filled gap defined by its neighboring layers. Spacers can bepositioned in the gap for structural support between the neighboringlayers and/or to control the width of the gap. Such a gap can provide adesirable index of refraction.

[0194]FIG. 23 shows an embodiment of a reflective display 800. Thedisplay 800 includes a reflective encapsulated display medium 610A,thin-film layers 830, 840 (such as capping layer 730 and transparentconductor 740), a low-index film 820, and a window 710 having aninterface 810 with an ambient air 750. The reflective encapsulateddisplay medium 610A includes a material portion 611A that is disposedbetween the reflective particles 614 and the low index-film 820.

[0195] Light from the ambient environment 750 and falling upon thereflective display undergoes refraction, described by Snell's law, as itpasses through the front air-window interface 810 into the window 710.As the light passes into the low-index film 820, it undergoes a secondrefraction away from the normal. Preferably, the thickness of thelow-index film 820 is selected to be larger than the longest wavelengthof visible light incident upon the display. For example, the low-indexfilm 820 can have a thickness of 2 μm or more.

[0196] The light then passes through one or more thin films, which caninclude a capping layer 830, such as silicon dioxide, silicon monoxide,or, more generally, SiO_(x), and a transparent conductor 840, such asindium tin oxide or conductive polymer. These films 830 are thin(preferably having a thickness in a range of approximately 100 nm toapproximately 200 nm) so the light ray does not refract substantiallyupon passing through the films 830. Preferably, these thin films arealso as transparent as possible.

[0197] The light ray then passes through the material portion 611A ofthe display medium 610A before striking the particles 614. The materialportion 611A can include portions of encapsulating structure, forexample, polymeric materials, of the display medium 610A. The portionmaterial 611A can be, for example, several micrometers thick, and canrefract the incident rays back toward the normal before the rays impingeon the particles 614. In embodiments of non-encapsulated displays, thematerial portion 611A can include portions of a barrier structure, orseparation structure surrounding a display medium (e.g. a fluidsuspended with particles).

[0198]FIGS. 24 and 25 illustrate the passage of light after scatteringfrom the reflective particles 614 in embodiments that include featuresof the above-described displays 600, 700A, 700B, 700C, 800. FIG. 24shows an embodiment of a display 600A that does not include a low-indexfilm, while FIG. 25 shows an embodiment of a display 900A that doesinclude a low-index film 720.

[0199] Referring to FIG. 24, in the display 600A without a low-indexrefractive layer, light incident on the display can experience the firstrefraction at the ambient environment-window interface 760 and then passwith little or no refraction through the thin-film transparent conductor740. If the refractive index of the front window 710 is roughly equal tothat of the binder 616 and capsules 612A, 612B, 612C, the light rayundergoes only a slight shift in angle due to refraction at the displaymedium 610 interface.

[0200] Referring to FIG. 25, the presence of the low-index refractivelayer 720 in the display 900A has only a small effect on the incidentpath of a ray of light prior to the ray striking the reflectiveparticles 614. Ambient light refracts away from the normal when itpasses from the window 710 into the low-index refractive layer 720, butalmost returns to its original path when it passes into the displaymaterial 610A. Thus, for a highly transparent low-index refractive layer720, light typically reaches the display material 610A with an angularintensity distribution close to that found for an optical stack thatdoes not include the low-index refractive layer 720.

[0201] The effect of the low-index refractive layer 720 in cooperationwith material disposed between the low-index refractive layer 720 andthe reflecting particles 614 can be greater, however, on the paths ofrays of light departing from the particles 614. The paths of reflectedlight are illustrated by arrows in FIGS. 24 and 25 for optical stacks,respectively, without and with the low-index refractive layer 720.

[0202] Referring again to FIG. 24, two electrodes 634A and 634B areshown to address different portions of the display medium 610, effectingtwo pixels 734A and 734B, respectively. Specifically, for example, theelectrode 634A may address capsules 612A and 612B, and a portion ofcapsule 612C; the electrode 634B may address the remaining portion ofcapsule 612C. As shown in FIG. 24, pixels 734A and 734B may exhibitdiffering optical result. In a monochromatic display, pixel 734A may beexhibiting a white state while the pixel 734B exhibits a black state. Ina multi-chromatic display, the two pixels 734A and 734B may beexhibiting two different colors such as red and blue, respectively.

[0203] In the embodiment shown in FIG. 24, the optical stack does notinclude the low-index refractive layer 720, and reflected light raystravel an essentially undisturbed path from the Lambertian reflectorportion (i.e. reflecting particles 910), and undergo internal reflection(e.g., light path AA) against the front air-window interface 760 (forexample, hundreds of micrometers distant from the reflective particles910. As illustrated, the light ray reflected by reflective particles 910in the capsule 612A and following the light path AA, is internallyreflected onto a pixel other than the originating pixel 734A. Theinternally reflected light may travel to nearby pixels where it iseither absorbed or re-reflected (which can produce, e.g., undesirableoptical cross-talk), or the light can travel out of the display edge.

[0204] Referring to FIG. 25, in an encapsulated display 900A, similar toFIG. 24, two electrodes 634A and 634B address different portions of thedisplay medium 610A, effecting two pixels 734A and 734B, respectively.The optical stack of the display 900A includes the low-index refractivelayer 720 and the material portion 611A of the display medium 610A thatis disposed between reflective particles 614 and the low-indexrefractive layer 720. The display materials that lie between the lowindex refractive layer 720 and the reflective particles 614 includes thematerial portion 611A of the encapsulated display medium 610A.

[0205] The material portion 611A of an encapsulated display may include,for example, capsule or cell material, binder material, and fluidmaterial. In alternative implementations of a display according toprinciples of the invention, additional layers of material can bedisposed between the material portion 611A and the low-index film 720,such as the thin-film coating 730 and transparent conductor 740. Inembodiments of non-encapsulated displays, the material portion 611A caninclude portions of a barrier structure, a wall structure, or separationstructure surrounding a display medium (e.g. a fluid suspended withparticles).

[0206] Still referring to FIG. 25, light undergoes several refractionevents as it exits the display. Exiting light may also undergo internalreflection, e.g., at the interface 770 between the thin film coating 730and the low-index refractive layer 720. According to principles of theinvention, most scattered light that internally reflects within theoptical stack reflects at the interface between the low-index refractivelayer 720 and the display materials that lie between the low indexrefractive layer 720 and the reflective particles 614.

[0207] The interface 770 between the thin-film coating 730 and thelow-index refractive layer 720 is preferably spaced at most a fewmicrometers (e.g., less than 2 μm) from the Lambertian reflector portion(e.g. reflecting particles 910). The combined thickness of the coating730, the transparent conductor 740 and the rest of the material portion611A can be selected to return most, e.g., more than 50%, of theinternally reflected light back to the same pixel (i.e., the pixel fromwhich the light scattered). For example, the light ray reflected byreflective particles 910 in the capsule 612A and following the lightpath AB, is internally reflected onto part of the originating pixel734A, and not to the adjacent pixel 734B. In effect, the proximity ofthe interface 770 between the low-index refractive layer 720 and thematerial portion 611A effectively forces internal reflection to takeplace closer to the reflective particle 614 where the originalreflection takes place. Understandably, a larger pixel can permit agreater thickness of these materials.

[0208] The thickness of the transparent conductor 740, for example, isapproximately 0.1 μm, to provide a balance between desirableconductivity and transparency. Thus, desirable internal reflection canbe dominated by the material portion 611A of the encapsulated displaymedium 610A that is disposed between the reflective particles 614 andthe low-index refractive layer 720. In some preferred implementations,the thickness of the material portion 611A is a few times a wavelengthof light being scattered by the particles 614, or greater.

[0209] In some preferred implementations, however, the distance betweenthe Lambertian reflector portion (particles 614) and the internalreflection interface is smaller than the longest wavelength of visiblelight to pass through the display.

[0210] With proper selection of indices of refraction and materialthicknesses, most of the light internally reflected within the materialportion 611A can be reflected back to the same pixel 734A (e.g. lightpath AB). The pixel size, for example, can be chosen to be several timesas large as the distance between the Lambertian reflector portion andthe internal reflection interface.

[0211] For example, pixels having a greatest dimension of 100 μm to 300μm, or smaller, can recapture scattered light that is internallyreflected within the material portion 611A. Thus, encapsulatedelectrophoretic displays and optical stacks, according to principles ofthe invention, can provide highly effective light recycling compared toat least some traditional display designs.

[0212] In implementations with desirable pixel cross-talk properties,the display materials that lie between the low index refractive layer720 and the reflective particles 614 have a thickness that causes mostlight that is scattered and internally reflected to return to the samepixel (i.e., the pixel from which it was scattered.) The thickness canbe chosen through use of geometrical considerations regarding the rangeof scattering angles that lead to internal reflection, the size andshape of a pixel, and the location of a scattering particle within thepixel. As described above, selection of a thickness of the displaymaterials that lie between the low index film 720 and the reflectiveparticles 614 that is much less than a pixel width can support thereturn to the same pixel of most scattered and internally reflectedlight.

[0213] Optical stacks of the invention generally do not degrade the“paper-like” optical performance of the Lambertian display. The outputlight still has an angular intensity distribution substantially as foundfor traditional stack designs, but the intensity magnitude is typicallygreater due to more efficient light recycling.

[0214] Referring again to FIGS. 22A-C, the advantages of the inventioncan be realized in other configurations as well. In the embodimentillustrated in FIG. 22B, for example, the low-index film 720 can beelectrically tuned so that it has desirable characteristics forinclusion in the electrical stack of the display material. In theembodiment illustrated in FIG. 22C, the patterned features of a colorfilter array, i.e., color filter resist materials 751, 752, 753 areincluded in the optical stack.

[0215] Optical stacks of the invention have other uses as well. Forexample, a low-index film 720 can be used as an enhancement film in theconstruction of LCD backlights. The films could be used to enhance thebrightness of emissive displays, for example, OLED, plasma, fieldemission, CRT, micro, and other emissive displays.

[0216] Further, aerogels can be included in a display, for example, toprovide an index of refraction that closely matches that of the ambientenvironment (e.g., air) and/or to utilize their exceptional thermalinsulation properties to help ruggedize a display against thermal shock.Moreover, a gain reflector of a liquid crystal display could utilizeprinciples of the invention to improve display brightness.

[0217] Although the invention has been described and illustrated aboveprimarily as used with encapsulated electrophoretic media, a variety ofother electro-optic media may be used in the displays of the presentinvention. The electro-optic medium could, for example, be a microcellelectrophoretic display, in which the charged particles and thesuspending fluid are not encapsulated within microcapsules but insteadare retained within a plurality of cavities formed within a carriermedium, typically a polymeric film. See, for example, InternationalApplications Publication No. WO 02/01281, and U.S. patent applicationPublication Ser. No. 2002/00,755,56, both assigned to Sipix Imaging,Inc.

[0218] The electro-optic medium could also be of the rotating bichromalmember type as described, for example, in U.S. Pat. Nos. 5,808,783;5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124;6,137,467; and 6,147,791 (although this type of display is oftenreferred to as a “rotating bichromal ball” display, the term “rotatingbichromal member” is preferred as more accurate since in some of thepatents mentioned above the rotating members are not spherical).

[0219] The electro-optic medium could also be an electrochromic medium,a preferred electrochromic medium being a nanochromic film comprising anelectrode formed at least in part from a semi-conducing metal oxide anda plurality of dye molecules capable of reversible color change attachedto the electrode; see, for example O'Regan, B., et al., Nature 1991,353, 737. Nanochromic films of this type are also described, forexample, in U.S. Pat. No. 6,301,038 and International ApplicationPublication No. WO 01/27690; the entire contents of this patent andapplication are herein incorporated by reference. For example,electrochromic particles can provide a color changing reflectivecomponent of a display via oxidation/reduction reactions.

[0220] The following portion of the Detailed Description describesvarious embodiments of materials that may be included in anelectrophoretic display medium of the electrophoretic displays of thepresent invention.

[0221] Useful materials for constructing an electrophoretic displaymedium, in particular for use in encapsulated electrophoretic displays,are discussed in further detail below. The discussion is organized intofive topics: particles; suspending fluid; charge control agents andparticle stabilizers; encapsulation; and binder material.

[0222] A. Particles

[0223] There is much flexibility in the choice of particles for use inelectrophoretic displays, as described above. For purposes of thisinvention, a particle is any component that is charged or capable ofacquiring a charge (i.e., has or is capable of acquiring electrophoreticmobility), and, in some cases, this mobility may be zero or close tozero (i.e., the particles will not move). The particles may be neatpigments, dyed (laked) pigments or pigment/polymer composites, or anyother component that is charged or capable of acquiring a charge.Typical considerations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light. The particles for use in the invention may furtherinclude scattering pigments, absorbing pigments and luminescentparticles. The particles may be retroreflective, such as corner cubes,or they may be electroluminescent, such as zinc sulfide particles, whichemit light when excited by an AC field, or they may be photoluminescent.Finally, the particles may be surface treated so as to improve chargingor interaction with a charging agent, or to improve dispersibility.

[0224] A preferred particle for use in electrophoretic displays of theinvention is Titania. The titania particles may be coated with a metaloxide, such as aluminum oxide or silicon oxide, for example. The titaniaparticles may have one, two, or more layers of metal-oxide coating. Forexample, a titania particle for use in electrophoretic displays of theinvention may have a coating of aluminum oxide and a coating of siliconoxide. The coatings may be added to the particle in any order.

[0225] The electrophoretic particle is usually a pigment, a polymer, alaked pigment, or some combination of the above. A neat pigment can beany pigment, and, usually for a light colored particle, pigments suchas, for example, rutile (titania), anatase (titania), barium sulfate,kaolin, or zinc oxide are useful. Some typical particles have highrefractive indices, high scattering coefficients, and low absorptioncoefficients. Other particles are absorptive, such as carbon black orcolored pigments used in paints and inks. The pigment should also beinsoluble in the suspending fluid. Yellow pigments such as diarylideyellow, hansa yellow, and benzidin yellow have also found use in similardisplays. Any other reflective material can be employed for a lightcolored particle, including non-pigment materials, such as metallicparticles.

[0226] Useful neat pigments include, but are not limited to, PbCrO₄,Cyan blue GT 55-3295 (American Cyanamid Company, Wayne, N.J.), CibacronBlack BG (Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G(Ciba), Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol BlackRBL (Ciba), Acetamine Blac, CBS (E. I. du Pont de Nemours and Company,Inc., Wilmington, Del.), Crocein Scarlet N Ex (du Pont) (27290), FiberBlack VF (DuPont) (30235), Luxol Fast Black L (DuPont) (Solv. Black 17),Nirosine Base No. 424 (DuPont) (50415 B), Oil Black BG (DuPont) (Solv.Black 16), Rotalin Black RM (DuPont), Sevron Brilliant Red 3 B (DuPont);Basic Black DSC (Dye Specialties, Inc.), Hectolene Black (DyeSpecialties, Inc.), Azosol Brilliant Blue B (GAF, Dyestuff and ChemicalDivision, Wayne, N.J.) (Solv. Blue 9), Azosol Brilliant Green BA (GAF)(Solv. Green 2), Azosol Fast Brilliant Red B (GAF), Azosol Fast OrangeRA Conc. (GAF) (Solv. Orange 20), Azosol Fast Yellow GRA Conc. (GAF)(13900 A), Basic Black KMPA (GAF), Benzofix Black CW-CF (GAF) (35435),Cellitazol BNFV Ex Soluble CF (GAF) (Disp. Black 9), Celliton Fast BlueAF Ex Conc (GAF) (Disp. Blue 9), Cyper Black IA (GAF) (Basic Blk. 3),Diamine Black CAP Ex Conc (GAF) (30235), Diamond Black EAN Hi Con. CF(GAF) (15710), Diamond Black PBBA Ex (GAF) (16505); Direct Deep Black EAEx CF (GAF) (30235), Hansa Yellow G (GAF) (11680); Indanthrene Black BBKPowd. (GAF) (59850), Indocarbon CLGS Conc. CF (GAF) (53295), KatigenDeep Black NND Hi Conc. CF (GAF) (15711), Rapidogen Black 3 G (GAF)(Azoic Blk. 4); Sulphone Cyanine Black BA-CF (GAF) (26370), ZambeziBlack VD Ex Conc. (GAF) (30015); Rubanox Red CP-1495 (TheSherwin-Williams Company, Cleveland, Ohio) (15630); Raven 11 (ColumbianCarbon Company, Atlanta, Ga.), (carbon black aggregates with a particlesize of about 25 μm), Statex B-12 (Columbian Carbon Co.) (a furnaceblack of 33 μm average particle size), and chrome green.

[0227] Particles may also include laked, or dyed, pigments. Lakedpigments are particles that have a dye precipitated on them or which arestained. Lakes are metal salts of readily soluble anionic dyes. Theseare dyes of azo, triphenylmethane or anthraquinone structure containingone or more sulphonic or carboxylic acid groupings. They are usuallyprecipitated by a calcium, barium or aluminum salt onto a substrate.Typical examples are peacock blue lake (CI Pigment Blue 24) and Persianorange (lake of CI Acid Orange 7), Black M Toner (GAF) (a mixture ofcarbon black and black dye precipitated on a lake).

[0228] A dark particle of the dyed type may be constructed from anylight absorbing material, such as carbon black, or inorganic blackmaterials. The dark material may also be selectively absorbing. Forexample, a dark green pigment may be used. Black particles may also beformed by staining latices with metal oxides, such latex copolymersconsisting of any of butadiene, styrene, isoprene, methacrylic acid,methyl methacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodiumstyrene sulfonate, vinyl acetate, chlorostyrene,dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate andN-(isobutoxymethacrylamide), and optionally including conjugated dienecompounds such as diacrylate, triacrylate, dimethylacrylate andtrimethacrylate. Black particles may also be formed by a dispersionpolymerization technique.

[0229] In the systems containing pigments and polymers, the pigments andpolymers may form multiple domains within the electrophoretic particle,or be aggregates of smaller pigment/polymer combined particles.Alternatively, a central pigment core may be surrounded by a polymershell. The pigment, polymer, or both can contain a dye. The opticalpurpose of the particle may be to scatter light, absorb light, or both.Useful sizes may range from 1 nm up to about 100 μm, as long as theparticles are smaller than the bounding capsule. In a preferredembodiment, the density of the electrophoretic particle may besubstantially matched to that of the suspending (i.e., electrophoretic)fluid. As defined herein, a suspending fluid has a density that is“substantially matched” to the density of the particle if the differencein their respective densities is between about zero and about two g/ml.This difference is preferably between about zero and about 0.5 g/ml.Useful polymers for the particles include, but are not limited to:polystyrene, polyethylene, polypropylene, phenolic resins, Du Pont Elvaxresins (ethylene-vinyl acetate copolymers), polyesters, polyacrylates,polymethacrylates, ethylene acrylic acid or methacrylic acid copolymers(Nucrel Resins—DuPont, Primacor Resins—Dow Chemical), acrylic copolymersand terpolymers (Elvacite Resins, DuPont) and PMMA. Useful materials forhomopolymer/pigment phase separation in high shear melt include, but arenot limited to, polyethylene, polypropylene, polymethylmethacrylate,polyisobutylmethacrylate, polystyrene, polybutadiene, polyisoprene,polyisobutylene, polylauryl methacrylate, polystearyl methacrylate,polyisobornyl methacrylate, poly-t-butyl methacrylate, polyethylmethacrylate, polymethyl acrylate, polyethyl acrylate,polyacrylonitrile, and copolymers of two or more of these materials.Some useful pigment/polymer complexes that are commercially availableinclude, but are not limited to, Process Magenta PM 1776 (Magruder ColorCompany, Inc., Elizabeth, N.J.), Methyl Violet PMA VM6223 (MagruderColor Company, Inc., Elizabeth, N.J.), and Naphthol FGR RF6257 (MagruderColor Company, Inc., Elizabeth, N.J.).

[0230] The pigment-polymer composite may be formed by a physicalprocess, (e.g., attrition or ball milling), a chemical process (e.g.,microencapsulation or dispersion polymerization), or any other processknown in the art of particle production. From the following non-limitingexamples, it may be seen that the processes and materials for both thefabrication of particles and the charging thereof are generally derivedfrom the art of liquid toner, or liquid immersion development. Thus anyof the known processes from liquid development are particularly, but notexclusively, relevant.

[0231] New and useful electrophoretic particles may still be discovered,but a number of particles already known to those skilled in the art ofelectrophoretic displays and liquid toners can also prove useful. Ingeneral, the polymer requirements for liquid toners and encapsulatedelectrophoretic inks are similar, in that the pigment or dye must beeasily incorporated therein, either by a physical, chemical, orphysicochemical process, may aid in the colloidal stability, and maycontain charging sites or may be able to incorporate materials whichcontain charging sites. One general requirement from the liquid tonerindustry that is not shared by encapsulated electrophoretic inks is thatthe toner must be capable of “fixing” the image, i.e., heat fusingtogether to create a uniform film after the deposition of the tonerparticles.

[0232] Typical manufacturing techniques for particles are drawn from theliquid toner and other arts and include ball milling, attrition, jetmilling, etc. The process will be illustrated for the case of apigmented polymeric particle. In such a case the pigment is compoundedin the polymer, usually in some kind of high shear mechanism such as ascrew extruder. The composite material is then (wet or dry) ground to astarting size of around 10 μm. It is then dispersed in a carrier liquid,for example ISOPAR® (Exxon, Houston, Tex.), optionally with some chargecontrol agent(s), and milled under high shear for several hours down toa final particle size and/or size distribution.

[0233] Another manufacturing technique for particles drawn from theliquid toner field is to add the polymer, pigment, and suspending fluidto a media mill. The mill is started and simultaneously heated totemperature at which the polymer swells substantially with the solvent.This temperature is typically near 100° C. In this state, the pigment iseasily encapsulated into the swollen polymer. After a suitable time,typically a few hours, the mill is gradually cooled back to ambienttemperature while stirring. The milling may be continued for some timeto achieve a small enough particle size, typically a few micrometers indiameter. The charging agents may be added at this time. Optionally,more suspending fluid may be added.

[0234] Chemical processes such as dispersion polymerization, mini- ormicro-emulsion polymerization, suspension polymerization precipitation,phase separation, solvent evaporation, in situ polymerization, seededemulsion polymerization, or any process which falls under the generalcategory of microencapsulation may be used. A typical process of thistype is a phase separation process wherein a dissolved polymericmaterial is precipitated out of solution onto a dispersed pigmentsurface through solvent dilution, evaporation, or a thermal change.Other processes include chemical means for staining polymeric lattices,for example with metal oxides or dyes.

[0235] B. Suspending Fluid

[0236] The suspending fluid containing the particles can be chosen basedon properties such as density, refractive index, and solubility. Apreferred suspending fluid has a low dielectric constant (about 2), highvolume resistivity (about 10{circumflex over ( )}15 ohm-cm), lowviscosity (less than 5 cst), low toxicity and environmental impact, lowwater solubility (less than 10 ppm), high specific gravity (greater than1.5), a high boiling point (greater than 90° C.), and a low refractiveindex (less than 1.2).

[0237] The choice of suspending fluid may be based on concerns ofchemical inertness, density matching to the electrophoretic particle, orchemical compatibility with both the electrophoretic particle andbounding capsule. The viscosity of the fluid should be low when you wantthe particles to move. The refractive index of the suspending fluid mayalso be substantially matched to that of the particles. As used herein,the refractive index of a suspending fluid “is substantially matched” tothat of a particle if the difference between their respective refractiveindices is between about zero and about 0.3, and is preferably betweenabout 0.05 and about 0.2.

[0238] Additionally, the fluid may be chosen to be a poor solvent forsome polymers, which is advantageous for use in the fabrication ofmicroparticles because it increases the range of polymeric materialsuseful in fabricating particles of polymers and pigments. Organicsolvents, such as halogenated organic solvents, saturated linear orbranched hydrocarbons, silicone oils, and low molecular weighthalogen-containing polymers are some useful suspending fluids. Thesuspending fluid may comprise a single fluid. The fluid will, however,often be a blend of more than one fluid in order to tune its chemicaland physical properties. Furthermore, the fluid may contain surfacemodifiers to modify the surface energy or charge of the electrophoreticparticle or bounding capsule. Reactants or solvents for themicroencapsulation process (oil soluble monomers, for example) can alsobe contained in the suspending fluid. Charge control agents can also beadded to the suspending fluid.

[0239] Useful organic solvents include, but are not limited to,epoxides, such as, for example, decane epoxide and dodecane epoxide;vinyl ethers, such as, for example, cyclohexyl vinyl ether and Decave®(International Flavors & Fragrances, Inc., New York, N.Y.); and aromatichydrocarbons, such as, for example, toluene and naphthalene. Usefulhalogenated organic solvents include, but are not limited to,tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene, carbon tetrachloride.These materials have high densities. Useful hydrocarbons include, butare not limited to, dodecane, tetradecane, the aliphatic hydrocarbons inthe Isopar® series (Exxon, Houston, Tex.), Norpar® (series of normalparaffinic liquids), Shell-Sol® (Shell, Houston, Tex.), and Sol-Trol®(Shell), naphtha, and other petroleum solvents. These materials usuallyhave low densities. Useful examples of silicone oils include, but arenot limited to, octamethyl cyclosiloxane and higher molecular weightcyclic siloxanes, poly (methyl phenyl siloxane), hexamethyldisiloxane,and polydimethylsiloxane. These materials usually have low densities.Useful low molecular weight halogen-containing polymers include, but arenot limited to, poly(chlorotrifluoroethylene) polymer (Halogenatedhydrocarbon Inc., River Edge, N.J.), Galden® (a perfluorinated etherfrom Ausimont, Morristown, N.J.), or Krytox® from DuPont (Wilmington,Del.). In a preferred embodiment, the suspending fluid is apoly(chlorotrifluoroethylene) polymer. In a particularly preferredembodiment, this polymer has a degree of polymerization from about 2 toabout 10. Many of the above materials are available in a range ofviscosities, densities, and boiling points.

[0240] The fluid must be capable of being formed into small dropletsprior to a capsule being formed. Processes for forming small dropletsinclude flow-through jets, membranes, nozzles, or orifices, as well asshear-based emulsifying schemes. The formation of small drops may beassisted by electrical or sonic fields. Surfactants and polymers can beused to aid in the stabilization and emulsification of the droplets inthe case of an emulsion type encapsulation. A preferred surfactant foruse in displays of the invention is sodium dodecylsulfate.

[0241] It can be advantageous in some displays for the suspending fluidto contain an optically absorbing dye. This dye must be soluble in thefluid, but will generally be insoluble in the other components of thecapsule. There is much flexibility in the choice of dye material. Thedye can be a pure compound, or blends of dyes to achieve a particularcolor, including black. The dyes can be fluorescent, which would producea display in which the fluorescence properties depend on the position ofthe particles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable, forming a solid absorbing polymer insidethe bounding shell.

[0242] There are many dyes that can be chosen for use in encapsulatedelectrophoretic display. Properties important here include lightfastness, solubility in the suspending liquid, color, and cost. Theseare generally from the class of azo, anthraquinone, and triphenylmethanetype dyes and may be chemically modified so as to increase thesolubility in the oil phase and reduce the adsorption by the particlesurface.

[0243] A number of dyes already known to those skilled in the art ofelectrophoretic displays will prove useful. Useful azo dyes include, butare not limited to: the Oil Red dyes, and the Sudan Red and Sudan Blackseries of dyes. Useful anthraquinone dyes include, but are not limitedto: the Oil Blue dyes, and the Macrolex Blue series of dyes. Usefultriphenylmethane dyes include, but are not limited to, Michler's hydrol,Malachite Green, Crystal Violet, and Auramine O.

[0244] C. Charge Control Agents and Particle Stabilizers

[0245] Charge control agents are used to provide good electrophoreticmobility to the electrophoretic particles. Stabilizers are used toprevent agglomeration of the electrophoretic particles, as well asprevent the electrophoretic particles from irreversibly depositing ontothe capsule wall. Either component can be constructed from materialsacross a wide range of molecular weights (low molecular weight,oligomeric, or polymeric), and may be pure or a mixture. In particular,suitable charge control agents are generally adapted from the liquidtoner art. The charge control agent used to modify and/or stabilize theparticle surface charge is applied as generally known in the arts ofliquid toners, electrophoretic displays, non-aqueous paint dispersions,and engine-oil additives. In all of these arts, charging species may beadded to non-aqueous media in order to increase electrophoretic mobilityor increase electrostatic stabilization. The materials can improvesteric stabilization as well. Different theories of charging arepostulated, including selective ion adsorption, proton transfer, andcontact electrification.

[0246] An optional charge control agent or charge director may be used.These constituents typically consist of low molecular weightsurfactants, polymeric agents, or blends of one or more components andserve to stabilize or otherwise modify the sign and/or magnitude of thecharge on the electrophoretic particles. The charging properties of thepigment itself may be accounted for by taking into account the acidic orbasic surface properties of the pigment, or the charging sites may takeplace on the carrier resin surface (if present), or a combination of thetwo. Additional pigment properties which may be relevant are theparticle size distribution, the chemical composition, and thelightfastness. The charge control agent used to modify and/or stabilizethe particle surface charge is applied as generally known in the arts ofliquid toners, electrophoretic displays, non-aqueous paint dispersions,and engine-oil additives. In all of these arts, charging species may beadded to non-aqueous media in order to increase electrophoretic mobilityor increase electrostatic stabilization. The materials can improvesteric stabilization as well. Different theories of charging arepostulated, including selective ion adsorption, proton transfer, andcontact electrification.

[0247] Charge adjuvants may also be added. These materials increase theeffectiveness of the charge control agents or charge directors. Thecharge adjuvant may be a polyhydroxy compound or an aminoalcoholcompound, which are preferably soluble in the suspending fluid in anamount of at least 2% by weight. Examples of polyhydroxy compounds whichcontain at least two hydroxyl groups include, but are not limited to,ethylene glycol, 2,4,7,9-tetramethyl-decyne-4,7-diol, poly(propyleneglycol), pentaethylene glycol, tripropylene glycol, triethylene glycol,glycerol, pentaerythritol, glycerol tris(12-hydroxystearate), propyleneglycerol monohydroxystearate, and ethylene glycol monohydroxystrearate.Examples of aminoalcohol compounds which contain at least one alcoholfunction and one amine function in the same molecule include, but arenot limited to, triisopropanolamine, triethanolamine, ethanolamine,3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, andtetrakis(2-hydroxyethyl)ethylene-diamine. The charge adjuvant ispreferably present in the suspending fluid in an amount of about 1 toabout 100 mg/g of the particle mass, and more preferably about 50 toabout 200 mg/g.

[0248] The surface of the particle may also be chemically modified toaid dispersion, to improve surface charge, and to improve the stabilityof the dispersion, for example. Surface modifiers include organicsiloxanes, organohalogen silanes and other functional silane couplingagents (Dow Corning® Z-6070, Z-6124, and 3 additive, Midland, Mich.);organic titanates and zirconates (Tyzor® TOT, TBT, and TE Series,DuPont, Wilmington, Del.); hydrophobing agents, such as long chain (C12to C50) alkyl and alkyl benzene sulphonic acids, fatty amines ordiamines and their salts or quaternary derivatives; and amphipathicpolymers which can be covalently bonded to the particle surface.

[0249] In general, it is believed that charging results as an acid-basereaction between some moiety present in the continuous phase and theparticle surface. Thus useful materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

[0250] Different non-limiting classes of charge control agents which areuseful include organic sulfates or sulfonates, metal soaps, block orcomb copolymers, organic amides, organic zwitterions, and organicphosphates and phosphonates. Useful organic sulfates and sulfonatesinclude, but are not limited to, sodium bis(2-ethylhexyl)sulfosuccinate, calcium dodecyl benzene sulfonate, calciumpetroleum sulfonate, neutral or basic barium dinonylnaphthalenesulfonate, neutral or basic calcium dinonylnaphthalene sulfonate,dodecylbenzenesulfonic acid sodium salt, and ammonium lauryl sulphate.Useful metal soaps include, but are not limited to, basic or neutralbarium petronate, calcium petronate, Co—, Ca—, Cu—, Mn—, Ni—, Zn—, andFe— salts of naphthenic acid, Ba—, Al—, Zn—, Cu—, Pb—, and Fe— salts ofstearic acid, divalent and trivalent metal carboxylates, such asaluminum tristearate, aluminum octoanate, lithium heptanoate, ironstearate, iron distearate, barium stearate, chromium stearate, magnesiumoctanoate, calcium stearate, iron naphthenate, and zinc naphthenate, Mn—and Zn— heptanoate, and Ba—, Al—, Co—, Mn—, and Zn— octanoate. Usefulblock or comb copolymers include, but are not limited to, AB diblockcopolymers of (A) polymers of 2-(N,N)-dimethylaminoethyl methacrylatequaternized with methyl-p-toluenesulfonate and (B) poly-2-ethylhexylmethacrylate, and comb graft copolymers with oil soluble tails of poly(12-hydroxystearic acid) and having a molecular weight of about 1800,pendant on an oil-soluble anchor group of poly(methylmethacrylate-methacrylic acid). Useful organic amides include, but arenot limited to, polyisobutylene succinimides such as OLOA 1200 and 3700,and N-vinyl pyrrolidone polymers. Useful organic zwitterions include,but are not limited to, lecithin. Useful organic phosphates andphosphonates include, but are not limited to, the sodium salts ofphosphated mono- and di-glycerides with saturated and unsaturated acidsubstituents.

[0251] Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule walls. For the typical highresistivity liquids used as suspending fluids in electrophoreticdisplays, nonaqueous surfactants may be used. These include, but are notlimited to, glycol ethers, acetylenic glycols, alkanolamides, sorbitolderivatives, alkyl amines, quaternary amines, imidazolines, dialkyloxides, and sulfosuccinates.

[0252] D. Encapsulation

[0253] An encapsulated electrophoretic display may take many forms.Encapsulation of the internal phase (e.g., electrophoretic particles andsuspending fluid) may be accomplished in a number of different ways. Thedisplay may comprise capsules dispersed in a binder. The capsules may beof any size or shape. The capsules may, for example, be spherical andmay have diameters in the millimeter range or the micrometer range, butis preferably from ten to a few hundred micrometers. The capsules may beformed by an encapsulation technique, as described below. Particles maybe encapsulated in the capsules. The particles may be two or moredifferent types of particles. The particles may be colored, luminescent,light-absorbing or transparent, for example. The particles may includeneat pigments, dyed (laked) pigments or pigment/polymer composites, forexample. The display may further comprise a suspending fluid in whichthe particles are dispersed.

[0254] The successful construction of an encapsulated electrophoreticdisplay requires the proper interaction of several different types ofmaterials and processes, such as a polymeric binder and, optionally, acapsule membrane. These materials must be chemically compatible with theelectrophoretic particles and fluid, as well as with each other. Thecapsule materials may engage in useful surface interactions with theelectrophoretic particles, or may act as a chemical or physical boundarybetween the fluid and the binder.

[0255] In some embodiments, the encapsulation of the or internal phasemay be performed by directly dispersing or emulsifying the internalphase into the binder (or a precursor to the binder materials) and aneffective “polymer-dispersed electrophoretic display” constructed. Insuch displays, voids created in the binder may be referred to ascapsules or microcapsules even though no capsule membrane is present.The binder dispersed electrophoretic display may be of the emulsion orphase separation type.

[0256] The internal phase can be encapsulated in a cell structure. Someexamples of cell structures are disclosed in U.S. Pat. No. to 6,327,072to Comiskey et al. A cell structure can include a single sheet or filmof polymeric material. The cell structure can be manufactured via any ofa variety of techniques, which include deposition, lithography, andembossing. Individual cells can have any of a variety of two-dimensionalshapes polygonal (as viewed along a normal to a plane of the cellstructure), which include round, square and. Individual cells can haveflat and or curved surfaces. For example, a cell wall can define apolygonal solid or a hemispherical solid.

[0257] Numerous suitable procedures for microencapsulation are detailedin both Microencapsulation, Processes and Applications, (I. E.Vandegaer, ed.), Plenum Press, New York, N.Y. (1974) and Gutcho,Microcapsules and Mircroencapsulation Techniques, Nuyes Data Corp., ParkRidge, N.J. (1976). The processes fall into several general categories,all of which can be applied to the present invention: interfacialpolymerization, in situ polymerization, physical processes, such ascoextrusion and other phase separation processes, in-liquid curing, andsimple/complex coacervation.

[0258] Numerous materials and processes should prove useful informulating displays of the present invention. Useful materials forsimple coacervation processes include, but are not limited to, gelatin,polyvinyl alcohol, polyvinyl acetate, and cellulosic derivatives, suchas, for example, carboxymethylcellulose. Useful materials for complexcoacervation processes include, but are not limited to, gelatin, acacia,carageenan, carboxymethylcellulose, hydrolyzed styrene anhydridecopolymers, agar, alginate, casein, albumin, methyl vinyl etherco-maleic anhydride, and cellulose phthalate. Useful materials for phaseseparation processes include, but are not limited to, polystyrene, PMMA,polyethyl methacrylate, polybutyl methacrylate, ethyl cellulose,polyvinyl pyridine, and poly acrylonitrile. Useful materials for in situpolymerization processes include, but are not limited to,polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde;water-soluble oligomers of the condensate of melamine, or urea andformaldehyde; and vinyl monomers, such as, for example, styrene, MMA andacrylonitrile. Finally, useful materials for interfacial polymerizationprocesses include, but are not limited to, diacyl chlorides, such as,for example, sebacoyl, adipoyl, and di- or poly-amines or alcohols, andisocyanates. Useful emulsion polymerization materials may include, butare not limited to, styrene, vinyl acetate, acrylic acid, butylacrylate, t-butyl acrylate, methyl methacrylate, and butyl methacrylate.

[0259] Capsules produced may be dispersed into a curable carrier,resulting in an ink which may be printed or coated on large andarbitrarily shaped or curved surfaces using conventional printing andcoating techniques.

[0260] In the context of the present invention, one skilled in the artwill select an encapsulation procedure and wall material based on thedesired capsule properties. These properties include the distribution ofcapsule radii; electrical, mechanical, diffusion, and optical propertiesof the capsule wall; and chemical compatibility with the internal phaseof the capsule.

[0261] The capsule wall generally has a high electrical resistivity.Although it is possible to use walls with relatively low resistivities,this may limit performance in requiring relatively higher addressingvoltages. The capsule wall should also be mechanically strong (althoughif the finished capsule powder is to be dispersed in a curable polymericbinder for coating, mechanical strength is not as critical). The capsulewall should generally not be porous. If, however, it is desired to usean encapsulation procedure that produces porous capsules, these can beovercoated in a post-processing step (i.e., a second encapsulation).Moreover, if the capsules are to be dispersed in a curable binder, thebinder will serve to close the pores. The capsule walls should beoptically clear. The wall material may, however, be chosen to match therefractive index of the internal phase of the capsule (i.e., thesuspending fluid) or a binder in which the capsules are to be dispersed.For some applications (e.g., interposition between two fixedelectrodes), monodispersed capsule radii are desirable.

[0262] An encapsulation procedure involves a polymerization between ureaand formaldehyde in an aqueous phase of an oil/water emulsion in thepresence of a negatively charged, carboxyl-substituted, linearhydrocarbon polyelectrolyte material. The resulting capsule wall is aurea/formaldehyde copolymer, which discretely encloses the internalphase. The capsule is clear, mechanically strong, and has goodresistivity properties.

[0263] The related technique of in situ polymerization utilizes anoil/water emulsion, which is formed by dispersing the electrophoreticcomposition (i.e., the dielectric liquid containing a suspension of thepigment particles) in an aqueous environment. The monomers polymerize toform a polymer with higher affinity for the internal phase than for theaqueous phase, thus condensing around the emulsified oily droplets. Inone especially useful in situ polymerization processes, urea andformaldehyde condense in the presence of poly(acrylic acid) (See, e.g.,U.S. Pat. No. 4,001,140). In other useful process, any of a variety ofcross-linking agents borne in aqueous solution is deposited aroundmicroscopic oil droplets. Such cross-linking agents include aldehydes,especially formaldehyde, glyoxal, or glutaraldehyde; alum; zirconiumsalts; and poly isocyanates. The entire disclosures of the 4,001,140 and4,273,672 patents are hereby incorporated by reference herein.

[0264] The coacervation approach also utilizes an oil/water emulsion.One or more colloids are coacervated (i.e., agglomerated) out of theaqueous phase and deposited as shells around the oily droplets throughcontrol of temperature, pH and/or relative concentrations, therebycreating the microcapsule. Materials suitable for coacervation includegelatins and gum arabic.

[0265] The interfacial polymerization approach relies on the presence ofan oil-soluble monomer in the electrophoretic composition, which onceagain is present as an emulsion in an aqueous phase. The monomers in theminute hydrophobic droplets react with a monomer introduced into theaqueous phase, polymerizing at the interface between the droplets andthe surrounding aqueous medium and forming shells around the droplets.Although the resulting walls are relatively thin and may be permeable,this process does not require the elevated temperatures characteristicof some other processes, and therefore affords greater flexibility interms of choosing the dielectric liquid.

[0266] Coating aids can be used to improve the uniformity and quality ofthe coated or printed electrophoretic ink material. Wetting agents aretypically added to adjust the interfacial tension at thecoating/substrate interface and to adjust the liquid/air surfacetension. Wetting agents include, but are not limited to, anionic andcationic surfactants, and nonionic species, such as silicone orfluoropolymer based materials. Dispersing agents may be used to modifythe interfacial tension between the capsules and binder, providingcontrol over flocculation and particle settling.

[0267] Surface tension modifiers can be added to adjust the air/inkinterfacial tension. Polysiloxanes are typically used in such anapplication to improve surface leveling while minimizing other defectswithin the coating. Surface tension modifiers include, but are notlimited to, fluorinated surfactants, such as, for example, the Zonyl®series from DuPont (Wilmington, Del.), the Fluorod® series from 3M (St.Paul, Minn.), and the fluoroakyl series from Autochem (Glen Rock, N.J.);siloxanes, such as, for example, Silwet® from Union Carbide (Danbury,Conn.); and polyethoxy and polypropoxy alcohols. Antifoams, such assilicone and silicone-free polymeric materials, may be added to enhancethe movement of air from within the ink to the surface and to facilitatethe rupture of bubbles at the coating surface. Other useful antifoamsinclude, but are not limited to, glyceryl esters, polyhydric alcohols,compounded antifoams, such as oil solutions of alkyl benzenes, naturalfats, fatty acids, and metallic soaps, and silicone antifoaming agentsmade from the combination of dimethyl siloxane polymers and silica.Stabilizers such as uv-absorbers and antioxidants may also be added toimprove the lifetime of the ink.

[0268] Other additives to control properties like coating viscosity andfoaming can also be used in the coating fluid. Stabilizers(UV-absorbers, antioxidants) and other additives which could proveuseful in practical materials.

[0269] E. Binder Material

[0270] The binder is used as a non-conducting, adhesive mediumsupporting and protecting the capsules, as well as binding the electrodematerials to the capsule dispersion. Binders are available in many formsand chemical types. Among these are water-soluble polymers, water-bornepolymers, oil-soluble polymers, thermoset and thermoplastic polymers,and radiation-cured polymers.

[0271] Among the water-soluble polymers are the various polysaccharides,the polyvinyl alcohols, N-methylpyrrolidone, N-vinylpyrrollidone, thevarious Carbowax® species (Union Carbide, Danbury, Conn.), andpoly-2-hydroxyethylacrylate.

[0272] The water-dispersed or water-borne systems are generally latexcompositions, typified by the Neorez® and Neocryl® resins (ZenecaResins, Wilmington, Mass.), Acrysol® (Rohm and Haas, Philadelphia, Pa.),Bayhydrol® (Bayer, Pittsburgh, Pa.), and the Cytec Industries (WestPaterson, N.J.) HP line. These are generally latices of polyurethanes,occasionally compounded with one or more of the acrylics, polyesters,polycarbonates or silicones, each lending the final cured resin in aspecific set of properties defined by glass transition temperature,degree of “tack,” softness, clarity, flexibility, water permeability andsolvent resistance, elongation modulus and tensile strength,thermoplastic flow, and solids level. Some water-borne systems can bemixed with reactive monomers and catalyzed to form more complex resins.Some can be further cross-linked by the use of a crosslinking reagent,such as an aziridine, for example, which reacts with carboxyl groups.

[0273] A typical application of a water-borne resin and aqueous capsulesfollows. A volume of particles is centrifuged at low speed to separateexcess water. After a given centrifugation process, for example 10minutes at 60× G, the capsules are found at the bottom of the centrifugetube, while the water portion is at the top. The water portion iscarefully removed (by decanting or pipetting). The mass of the remainingcapsules is measured, and a mass of resin is added such that the mass ofresin is between one eighth and one tenth of the weight of the capsules.This mixture is gently mixed on an oscillating mixer for approximatelyone half hour. After about one half hour, the mixture is ready to becoated onto the appropriate substrate.

[0274] The thermoset systems are exemplified by the family of epoxies.These binary systems can vary greatly in viscosity, and the reactivityof the pair determines the “pot life” of the mixture. If the pot life islong enough to allow a coating operation, capsules may be coated in anordered arrangement in a coating process prior to the resin curing andhardening.

[0275] Thermoplastic polymers, which are often polyesters, are molten athigh temperatures. A typical application of this type of product ishot-melt glue. A dispersion of heat-resistant capsules could be coatedin such a medium. The solidification process begins during cooling, andthe final hardness, clarity and flexibility are affected by thebranching and molecular weight of the polymer.

[0276] Oil or solvent-soluble polymers are often similar in compositionto the water-borne system, with the obvious exception of the wateritself. The latitude in formulation for solvent systems is enormous,limited only by solvent choices and polymer solubility. Of considerableconcern in solvent-based systems is the viability of the capsuleitself—the integrity of the capsule wall cannot be compromised in anyway by the solvent.

[0277] Radiation cure resins are generally found among the solvent-basedsystems. Capsules may be dispersed in such a medium and coated, and theresin may then be cured by a timed exposure to a threshold level of veryviolet radiation, either long or short wavelength. As in all cases ofcuring polymer resins, final properties are determined by the branchingand molecular weights of the monomers, oligomers and crosslinkers.

[0278] A number of “water-reducible” monomers and oligomers are,however, marketed. In the strictest sense, they are not water soluble,but water is an acceptable diluent at low concentrations and can bedispersed relatively easily in the mixture. Under these circumstances,water is used to reduce the viscosity (initially from thousands tohundreds of thousands centipoise). Water-based capsules, such as thosemade from a protein or polysaccharide material, for example, could bedispersed in such a medium and coated, provided the viscosity could besufficiently lowered. Curing in such systems is generally by ultravioletradiation.

[0279] While the invention has been particularly shown and describedwith reference to specific preferred embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention as defined by the appended claims. For example, adisplay element may include more than one optical biasing element, forexample, an element embedded in a binder as well as a layer of materialadded beneath the display medium.

What is claimed is:
 1. A display device comprising: a window layerhaving a refractive index that is greater than a refractive index of anambient environment; a plurality of reflective particles for scatteringlight received from the ambient environment; a material portion disposedbetween the plurality of reflective particles and the window layer; anda refractive layer disposed between the material portion and the windowlayer, and having a refractive index that is less than the refractiveindex of the window layer and less than a refractive index of thematerial portion.
 2. The display device of claim 1, further comprisingat least one film layer disposed between and in contact with thematerial portion and the refractive layer.
 3. The display device ofclaim 2, wherein a portion of the plurality of reflective particlesbelongs to a pixel of the display device, and the material portion andthe at least one film layer have a combined thickness such that morethan half of light scattered by the portion of the plurality ofreflective particles and internally reflected returns to the same pixel.4. The display device of claim 3, wherein the combined thickness is lessthan 10 micrometers.
 5. The display device of claim 3, wherein thecombined thickness is less than 3 micrometers.
 6. The display device ofclaim 2, wherein the at least one film layer comprises an electricallyconductive layer.
 7. The display device of claim 2, wherein each of theat least one film layer has a thickness in a range of 0.05 to 0.30micrometer.
 8. The display device of claim 1, wherein the plurality ofreflective particles provide a Lambertian distribution of scatteredlight.
 9. The display device of claim 1, further comprising anencapsulating structure that encapsulates the plurality of reflectiveparticles, wherein the material portion is a portion of theencapsulating structure.
 10. The display device of claim 9, wherein theencapsulating structure comprises at least one structure selected fromthe group consisting of a cell structure, a capsule membrane, and abinder.
 11. The display device of claim 1, wherein the material portioncomprises a layer defining a wall that contains the plurality ofreflective particles.
 12. The display device of claim 1, wherein therefractive index of the refractive layer is closer to the refractiveindex of the ambient environment than to the refractive index of thematerial portion.
 13. The display device of claim 1, wherein therefractive index of the material portion is greater than 1.4.
 14. Thedisplay device of claim 1, wherein the refractive layer comprises eithera vacuum or a gas-filled gap.
 15. The display device of claim 1, whereinthe refractive layer comprises a porous material.
 16. The display deviceof claim 1, wherein the ambient environment comprises air, and therefractive index of the refractive layer is in a range of 1.0 to 1.2.17. The display device of claim 1, wherein the refractive layercomprises a composite material.
 18. The display device of claim 1,wherein the refractive layer has a thickness greater than the longestwavelength of visible light incident upon the display.
 19. The displaydevice of claim 18, wherein the refractive layer has a thickness greaterthan 1 micrometer.
 20. The display device of claim 1, wherein therefractive index of the refractive layer is less than 1.3.
 21. Thedisplay device of claim 1, wherein the reflective particles comprise atleast one of electrophoretic particles, rotating bichromal members, andelectrochromic particles.
 22. A method for making a display device, themethod comprising: providing a window layer having a refractive indexthat is greater than a refractive index of an ambient environment;providing a plurality of reflective particles for scattering lightreceived from the ambient environment; providing a material portiondisposed between the plurality of reflective particles and the windowlayer; and providing a refractive layer disposed between the materialportion and the window layer, and having a refractive index that is lessthan the refractive index of the window layer and less than a refractiveindex of the material portion.
 23. The method of claim 22, furthercomprising selecting a thickness of the material portion to cause mostinternally reflected scattered light to return to a same pixel.